There are actually a few of them alongside Postmarket. LineageOS and its various forks, Ubuntu Touch, KDE Plasma Mobile, Graphene, Replicant… the list goes on!
Unfortunately, I was playing around with them a few years ago and Android phones seem to be a POS to try and switch the OS on. But, it’s clearly done. I’m thinking about at least popping one of them onto an older phone or tablet.