What most stands out to me is your statement that "I’ve been a member of just about every mainstream church at one time or another. " Which means that not only is it easy to JOIN a “mainstream church”, it’s easy to LEAVE one as well.
But there is no (longer) a way to “formally defect” from the Catholic Church. Once you’re in, you’re in. All of the obligations of being a Catholic continue, as well as the penalties of NOT meeting those obligations, even if a person claims “I’m an ex-Catholic”. So, an “ex-Catholic” who marries in a courthouse, on a beach, in a Baptist church, Vegas wedding chapel, etc., is automatically entering an invalid marriage due to “lack of form”. An “ex-Catholic” is still obligated to attend Mass, and could be in mortal sin for not doing so. (But people who never were Catholics at all aren’t required to meet such standards.)
Now, if the Catholic faith isn’t true, then it doesn’t really matter from the POV of the “ex-Catholic” that they are still considered to be members of the Church, but obviously, those in the Church believe the faith is true. So, even without invoking the concept of “apostasy” which is a sin by itself, one can argue that an “ex-Catholic” even at greater risk of damnation, than those who were never Catholics in the first place!
This is the same reason that priests are not supposed to baptize infants as Catholics who don’t have a serious chance of actually growing up as practicing Catholics (although it seems that in practice, especially under Pope Francis, most priests are inclined to give the benefit of the doubt and perform the baptism, even if the parents are unmarried, are C+E Catholics, etc.)
To me, this is one of the major reasons that have kept me from pursuing the Church further. I just don’t feel ready to sign up for life. But I totally understand the reasoning behind why it’s not easy to become a Catholic.