The real question is, "Where are the free foundry PDKs?".
Because you can make whatever kind of polygon stew you
want, but it comes to nothing if you can't get it onto the reticle
(and foundries can be very rigid about the "golden" tapeout
tool set) or if it's all on you to make the layers, device
primitives, models and verification decks.
You might spend some time working backwards from that
viewpoint, and see if any free tools support the foundry
you intend to use, and vice versa.