“I think, the direct meaning is that the kingdom of God is when demons are cast out,”
Did the kingdom of God come in the person of Jesus Christ or did it come, like in the Old Testament, on Jesus when He was healing, exorcising, teaching etc.? In other words, is the exorcism the evidence that it has come or does it come and go as gifts are exercised or good works are done?
I can only repeat what the scripture and the Catholic Liturgy say. There are references to the Kingdom as something inside the believer, and there are references to it as a Second Coming of Christ.
During Mass, the priest calls us, “let us pray for the coming of the Kingdom as Jesus taught us”, and in response we say “Our Father”. Exorcisms are an extreme example of the daily work of sanctification to which we are called, individually and communally: “... Thy kingdom come, Thy will be done on earth as it is in heaven ... deliver us from [the] evil [one]”. The individual work of the Holy Spirit is not separated from the redemptive work of Christ in this prayer, which is fittingly addressed to the Father.
The immediate context of Matthew 12:28 reflects this duality of the individual and the communal. The context preceding the verse is individual acts of exorcisms. But following the verse, the context turns to the redemptive work of Christ Who bound Satan (v. 29, and also see the discourse on the “sign of Jonas” that follows).
I have a feeling I am not answering to your satisfaction, but not out of avoidance, feel free to press and elaborate. Perhaps, this has to do with the lack of shared eschatology. As Catholic, I believe that the tribulation is what we are in for the past 2,000 years; we do not think that it is a period that is yet to start. We are Church Militant, and we combat Satan, every day, internally and communally.