Deacons can preside over Catholic marriages, yes.
They can also perform all of the educative functions, give the Cana classes.
The only things that can only be done by an ordained priest (or bishop) are the consecration of the communion host (deacons, and even lay people can distribute it), pronouncing absolution in confession (deacons can HEAR confessions, but they can't pronounce absolution), anointing the sick in the sacrament of extreme unction.
Only a bishop can perform the sacraments of ordination of clergy and confirmation.
Marriage is a special case, in that, as you say, it is really the marrying couple who perform the sacrament. For it to be recognized by the Church, of course, it needs to meet canon law requirements. A deacon can preside.
Baptism is a really special case. ANY Christian can baptise another, technically speaking. Of course, again, to be valid (as far as the Church is concerned) it needs to be done under certain conditions, etc. It SHOULD be done by the clergy in a Church, but in an emergency you yourself can baptize. I really don't know if Deacons normally perform baptismal duties in the Catholic Church or not. There are, no doubt, canonical rules about this, but those rules are akin to priestly celibacy: they are binding, but it is because of order and discipline, not because of the specific attributes of divine power moving through the sacerdotal office. By contrast, in the most dire emergency a layman or a deacon CAN'T consecrate the communion host. It isn't that he's barred from doing it, it is that it won't work. Or we presume it won't work. God might choose to actually come down and transubstantiate anyway, but we have no assurance of that at all, and it is tempting fate to try it.
Looked at this way, there are only three things that a parish priest can do that a deacon or layman can't: consecrate the host, pronounce absolution at confession and perform extreme unction.
Now, my view of these things is that it is really the sacraments, above all, that bring men close to God. Bible reading is good, but Jesus didn't say to do that. He said to partake of him in bread and wine. Reading Scripture is a nice adjunct to faith and learning, but it is unneccessary. Taking communion IS the faith. To take communion in a state of sin is perilous. Thus, confession IS the faith, and is necessary to communion. Jesus was baptized, and demanded it of his disciples. And Jesus anointed the sick and demanded it of his disciples. All of this, too, IS the faith.
My personal view is that the role of the priest is to be the sacred vessel that carries the sacraments of God with him and dispenses them. In the process of doing that, there are many teaching moments. But the role of good, bad, or indifferent teacher is very secondary: the priest primary role is to be the one who, with certitude, can invoke the presence of God and everyone be absolutely certain that God is there and present. He might be anyway. He's God. The priest provides certitude. Which is why the priest, injecting himself into the presence of God all day, must be as pure as the beaten golden vessel that bears the host, and as pure as the silver chalice that holds the wine.
He is a rare species, the priest, the man who CAN keep himself pure and stainless, and keep washing off the spots by confession, so as to be a chalice of God for others. But that, I think, is his job.
And given the low number of vocations there really are, just doing THAT, meaningfully, should be (in my view) what priests are about. Priests should be made free to deliver the sacraments everywhere, all the time. Other people should handle all of the administration and finances and management and decision-taking, so that priests can be out there bringing God to earth before the anointed sick, and hearing confessions at all times and places, forgiving sins and dispensing the bread of life.
Brothers and sisters and deacons should run the Church.
Priests should be free to stand in personam Christi to the laity all the time, and should do so. People need God directly more than they need good parish management, and anyone can manage the parish, but only the priest can with certitude call God and his angels down from heaven into a specific room at a specific time and place. So he should do that and nothing else. And bishops should be free to stand in personam Christi to priests all the time.
We are accustomed to linking the administrative and monarchic power with the sacerdotal office...because the priest and bishop are the ones who hold the keys to the sacraments, they are the most important, and therefore should take all of the important decisions. But this is an error.
Jesus did not carry the money bag. That was Judas' job. Jesus didn't procure the bread and supplies and go out and negotiate for it. That was the job of other disciples. Jesus was the fount of wisdom and holiness. Others carried the mundane burdens, so that Jesus could do that. And in doing this, THEY WERE BLESSED.
Because the priest stands so often in the presence of God, God has to want him there. That's why a priest has to be called by God, and why nobody should force himself into that office by sheer willpower or desire for anything. God knows who he wants to be seeing and be called down by, he lets those men know, and when he does, he disposes them to follow. The whole structure of the Church should be about UNBURDENING those whom God has offer his sacraments from all of those things that other men and women CAN do, so that God's chosen men can do the sacramental things more widely and easily than they can when burdened with the world.
The ideal parish would be a place where all the sermons are given by powerful orators who are deacons, all the baptisms and marriages are performed by inspiring deacons, all of the administration of money, the school, construction and charity is performed by nuns and brothers and deacons and lay people, all of the religious instruction is performed by deacons, and where the priest spends his whole time visiting the sick in hospital, hearing confessions and pronouncing absolutions, and standing before God with the host at mass. The priest would do well an effaced creature of God, bringing God's sacraments to far more people, refreshing the people in spirit, while the great deacons lead the congregations in thought.
So think I.
Unfortunately, it is very difficult to unthink the natural tendency to concentrate power of decision with importance of office. The nature of the sacred - and priests are sacred instruments - should be PRESERVED FROM the profane which should properly done by loving experts in the field from the parish congregation, or devoted brothers and sisters.
They do; deacons are "ordinary ministers" of Sacrament of Baptism.
I'm reminded of the office of Arch-Deacon, and how difficult it was to think of a deacon in those terms. Bishops probably don't like the idea of an Archdeacon because they want to be the boss of the priests, and in a non-dysfunctional parish, the top deacon really should be the boss of the building, school, and personnel. When I was very new, the Pastor introduced my choirmaster to the congregation one day, because he tends to hide behind some potter plants (effacing himself). Anyway, he is the Director of Liturgy and Worship and the Pastor said "which technically makes him my boss... ha ha ha ha ha." To me, the following scripture sums the whole thing up: Act 6:2 "Then the twelve called the multitude of the disciples [unto them], and said, It is not reason that we should leave the word of God, and serve tables. Act 6:3 Wherefore, brethren, look ye out among you seven men of honest report, full of the Holy Ghost and wisdom, whom we may appoint over this business. Act 6:4 But we will give ourselves continually to prayer, and to the ministry of the word. Act 6:5 And the saying pleased the whole multitude: and they chose Stephen, a man full of faith and of the Holy Ghost, and Philip, and Prochorus, and Nicanor, and Timon, and Parmenas, and Nicolas a proselyte of Antioch: Act 6:6 Whom they set before the apostles: and when they had prayed, they laid [their] hands on them. Act 6:7 And the word of God increased; and the number of the disciples multiplied in Jerusalem greatly; and a great company of the priests were obedient to the faith. Act 6:8 And Stephen, full of faith and power, did great wonders and miracles among the people. New American Standard Bible © 1995 Lockman Foundation OK, one more thing. I think the diaconate should continue to increase in power and prestige. I also think the Religious Life for women should be glorified and presented to the believers, I hate to use the word "marketed", but I will. Because women need to understand their value in the church and have that celebrated. People make too big a deal out of the power to break the bread and hear the confession. Being a priest is more of a sacrifice than a benefit, or should be. But being a Sister, like a deacon, or a Brother, is a great thing, but seems to the average lay person like something drab and boring.