You will not find the word ‘Sabbath’ anywhere in Rm 14. The entire chapter is about fasting, one may fast on Monday and another may fast on Tuesday, etc, etc and we are to respect whatever day one chooses as their ‘fast to the LORD’. The Sabbath of God is a feast day, not a fast day. It was Roman Cristendom that changed His feast day into a fast day according to her council canons.
Fasting was a great controversy back then as the religious elites of Judaism had made laws setting aside specific days for fasting and anyone who did not follow their oral laws, their ‘dogma’ were subject to being fined and possibly even kept from entering the Temple of God as a punishment.
The big issue with the “Sabbath is none of us ultimately know if the Saturday or Sunday that some us may call the Sabbath (or not)...is actually the Sabbath. It wouldn’t surprise me that God’s resting day occurred on what we would call..a Thursday! God covenanted via the Decalog that the 7th day should be kept as the Sabbath...the 7th day would have been what ever the Hebrews called it based on time keeping used in Egypt before they left it. There was no mention of Abraham and the earlier Patriarchs keeping a special day called the Sabbath...none that I saw in the Bible.
As for the record lets actually look at Romans 14:
14 Accept the one whose faith is weak, without quarreling over disputable matters. 2 One person’s faith allows them to eat anything, but another, whose faith is weak, eats only vegetables. 3 The one who eats everything must not treat with contempt the one who does not, and the one who does not eat everything must not judge the one who does, for God has accepted them. 4 Who are you to judge someone else’s servant? To their own master, servants stand or fall. And they will stand, for the Lord is able to make them stand.
5 One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind. 6 Whoever regards one day as special does so to the Lord. Whoever eats meat does so to the Lord, for they give thanks to God; and whoever abstains does so to the Lord and gives thanks to God. 7 For none of us lives for ourselves alone, and none of us dies for ourselves alone. 8 If we live, we live for the Lord; and if we die, we die for the Lord. So, whether we live or die, we belong to the Lord. 9 For this very reason, Christ died and returned to life so that he might be the Lord of both the dead and the living.
10 You, then, why do you judge your brother or sister[a]? Or why do you treat them with contempt? For we will all stand before God’s judgment seat. 11 It is written:
“‘As surely as I live,’ says the Lord,
‘every knee will bow before me;
every tongue will acknowledge God.’”[b]
12 So then, each of us will give an account of ourselves to God.
13 Therefore let us stop passing judgment on one another. Instead, make up your mind not to put any stumbling block or obstacle in the way of a brother or sister. 14 I am convinced, being fully persuaded in the Lord Jesus, that nothing is unclean in itself. But if anyone regards something as unclean, then for that person it is unclean. 15 If your brother or sister is distressed because of what you eat, you are no longer acting in love. Do not by your eating destroy someone for whom Christ died. 16 Therefore do not let what you know is good be spoken of as evil. 17 For the kingdom of God is not a matter of eating and drinking, but of righteousness, peace and joy in the Holy Spirit, 18 because anyone who serves Christ in this way is pleasing to God and receives human approval.
19 Let us therefore make every effort to do what leads to peace and to mutual edification. 20 Do not destroy the work of God for the sake of food. All food is clean, but it is wrong for a person to eat anything that causes someone else to stumble. 21 It is better not to eat meat or drink wine or to do anything else that will cause your brother or sister to fall.
22 So whatever you believe about these things keep between yourself and God. Blessed is the one who does not condemn himself by what he approves. 23 But whoever has doubts is condemned if they eat, because their eating is not from faith; and everything that does not come from faith is sin.[c]”
There is only an oblique reference to fasting in verse 5 of that chapter about fasting; that is,”FEASTING or Abstaining” as conscience may dictate. Verse 5 refers not necessarily about feasting or fasting, but about special days to be kept based on one’s on conscience on the matter. Since even by your own words, the Sabbath is a “feast” day; then, according to Romans 14, we shouldn’t even be arguing over the technicalities of WHEN or how for that matter or what should be consumed or not consumed!
Keeping the Sabbath on a Saturday or a Sunday is a Faith thing, a conscience thing. Roman’s 14 doesn’t support your position at all!