I know it's off-topic from the SC, but I am going to follow it anyway.
First of all, a recent Pew Study indicated that 56% of US Catholics believe there should be legal in all or most cases. (Pew, 2020). So, if you are defining a liberal Catholic as someone who believes abortion should be legal, then "liberal" Catholics are massively underrepresented on the SC. I know that's not the perception because pro-life Catholics use their religion to bolster their position, while pro-choice Catholic politicians do not, but the fact is most actual, real life Catholics support abortion rights. And there are plenty who vote Democrat, with Gallup estimating that 52% voted Dem in 2020. (Gallup).
Secondly, regarding the purge, you're not entirely wrong (if you replace "Catholic" with "pro-lifer"), but it misses the main story. Local politics have become national. Because of that, it is harder for local candidates to follow regional preferences if they conflict with national party preferences. Case in point, I volunteered for a mayoral candidate (Mello) in Omaha who had some pretty moderate views on abortion. He got some press. National pro-choice organizations raised a stink and Tom Perez backed off him. Whether that cost him the election, I don't know. His campaign wasn't great and he was going against an incumbent. So, sure, there is that dynamic. But you could say the same about the influence Grover Norquist or the NRA has on local Republican candidates.
And a thirdly, you shouldn't just attribute changes in the last 30 years to the Democrats. The Church changed. I graduated from a Benedictine high school in 1990. I certainly knew liberal priests. I remember activist nuns who visited to talk about protesting nukes. Certainly there were conservative priests, including ones that protested abortion clinics. I remember paging through a liberal Catholic trade magazine (for the lack of a better term) that a priest had left out. My impression was that it was a cross section of politics. I don't get the impression it's that way now. My impression is that Conference of Catholic Bishops has been effective in pushing their bullshit down to the high school level. Hopefully I am wrong.