Hey there!
Thanks for bringing this up. I do have plans to support PieFed in the future - I can't promise when it will happen, but I'm working on laying down the groundwork to better support other APIs (whether it be Lemmy, PieFed, or possibly another platform that shows up in the future).
Doing this will take some time though, as Thunder's internal logic was (and is currently still) heavily tied to Lemmy's APIs. The good news is that the upcoming v0.7.0 release will bring us a step closer to this goal. My (optimistic) goal is to have basic support for PieFed and the upcoming Lemmy v1.0 API by the end of the year.