Well idk about a 100% free phone os like trisquel but there is GrapheneOS but that one only works with Pixel phones. And some banking apps don't work so you have to check if your banking app works if you use a banking app, and google pay + NFC i'm pretty sure does not work. But everything else should work out of the box. There's also lineageOS that works on more phones then just Pixel phones so that might be an alternative.