The way you phrase the question pre-supposes something negative from the Soviet days. The Russian Church gave countless martyrs during the Soviet period. Were there those in the Church that succumbed to temptation. Sure there were, just like there are in all persecutions. But the Church commemorates the martyrs and not the persecutors. You make it sound like the Russian Church is an apologist for the persecutions. You can tell a lot by looking at who the Saints are that the Russian Church has canonised (The Royal Family, Patriarch Tikhon etc). Also I'm not sure how the Russian Church differs from any of the other local Churches that found themselves under a Communist Regime. Based on your logic I should have separated from the Serbian Patriarchate.
Yes, apostasy is something negative, are we not to expect repentance from apostasy? The Russian Church gave countless martyrs, we agree on this statement, though we have a disagreement who the Russian Church consists of, the apostates are not included with them. Those who willingly worked with the state in the persecution of the faithful are not Orthodox. The priests/bishops who were appointed because of their loyalty to the state, are not Orthodox.
Sergius' declaration was in 1927, he and the others, and the later state appointed clergy proceeded to work with the state in the persecution of the faithful, and you want to say that everything is fine, because his successors canonized the people they had a hand in persecuting. Am I understanding you correctly?
I'm unfamiliar with the situation in Serbia, but if the situation was identical, then yes, I would follow the leaders that separated, but I'm not you, nor am I in your shoes to judge. I cannot say that a state puppet is a legitimate church. The Church must safeguard it's autonomy.