So far the only gods we
know we can bind ourselves to are Keros, Lumia and Nareva. Velun's a 'maybe' based on his being included in the wiki entry on perma-TFs (written by Tobs) but that's probably up in the air since I don't think they've settled on what Velun looks like now, so any TF is likely to be equally in flux and I don't think anyone on the team has talked about a Velun TF outside of that wiki page, unless it's on Discord somewhere. The other three gods all represent existing races with no obvious TF options (Tira is human, Mallach a catboy and Sorra a harpy) so they're unlikely. Savin mentioned two of those three in
this older post.
Nareva is the patron of mages and scholars so if Berwyn worships anybody, it's probably her; I'm pretty sure he actually swears by her name at least once. Arona is
definitely a Lumian, her older sister is even a priestess.