But it was not Constantine that made those decisions but rather the Bishops that gathered in that council.
No, Constantine did not have a vote. Neither did the Soviet Union's Politburo have a vote in the Supreme Soviet, the governing body that actually passed the laws. And yet, all the Supreme Soviet did was to rubber-stamp the decisions already reached by the Politburo (though Wikipedia notes that starting in the 1950's the Supreme Soviet started to slowly accrue some real power for itself).
Constantine called for the Council, convened the Council, and attended the Council, obviously
with the soldiers who formed the Pretorian Guard. He had an agenda in the proceedings and it strains credulity that his presence and his armed guards' presence did not unduly influence the bishops' votes.
We would need to consult historical records to piece together what had actually happened. But the fact still stands that going into the First Council both trinitarianism and non-trinitarianism were sizable factions and the Council did indeed decide for trinitarianism. I do believe that this is what Greatest Am I was referring to, which is the reason for my reply to you.
In addition, Christianity has never been monolithic. There have always been schisms and sects.
Yes, indeed. And that and more had already been happening long before going into the ecumenical councils. What the councils sought to do was to unify all those separate sects into one monolithic church (one article cites this as Constantine's primary agenda). And, of course, the splintering started anew almost immediately.