I have not read every post, so please ignore this if it’s redundant.
Having heard Protestants heatedly dispute differences in belief to the point of declaring each other “hell bound” and such, I have a lot of difficulty agreeing with the OP. Differences that may seem minute to some have a major meaning to some of the various churches. I know that where I live, non-denominational churches spring up like mushrooms after a spring rain, and those schisms are all based on some different interpretation of something in the bible. Maybe they really don’t seem major to me, but they do to them, and they’re sufficiently significant to them to prevent them from finding any affiliation to which they feel they can adhere.
There are lots of dissident Catholics, and there’s no denying that. But there is not a lot of dispute over what the Church actually teaches, because it has documented it in great detail. One can criticize the Catholic Church for excess in that regard, I suppose, but one can look in vain forever for what, e.g., the Southern Baptist churches hold, beyond a very small core of beliefs. Beyond that minimalist formulation, it’s just whatever anyone wants to believe. Look it up, their core beliefs are very thin, but the differences that are outside the core beliefs are potentially massive, presumably based on the bible, and are the very thing that causes schisms.
I have, in the past, become a participant in Protestant “church fights” and they’re anything but pretty, because they’re usually about who gets the church building and who has to leave and go build their own. It gets very tricky, because what might seem to some to be very small doctrinal differences are often that which determines the outcome of who is a member of the church and who isn’t. And all of them are supposedly based on inerrant scripture.
I remember one in which, for example, the dispute was over whether the bible allowed for musical instruments to be played with congregational singing and, if so, what kind. Of course, that was between two competing versions of the Church of Christ, whose individual churches claim to be non-denominational anyway. Each had its own biblical authority for its proposition, and the conflict was heartfelt. As it turned out, one was insufficiently faithful to the seeming intention of its local founders who donated the land on which the church was built, and had to pack up and ship out.
I have heard Protestants of one kind declare other kinds to be “cults”, by which they mean essentially pagan and condemned by God. Many Protestant divisions are no joke.