Jellyfin exists on various hardware as an optional install so it seems likely FCast would work on that distribution model.

I am more troubled by how you want to get the Cast button into Netflix, YouTube, etc.

I am not counting on this to happen quickly if at all. Though if FCast is the thing everyone has access to (regardless of Apple TV, Fire TV, Android TV, etc) then maybe eventually it will happen. If you have a raspberry pi, and old computer or an old phone you aren't using, you can easily convert it into a FCast receiver. Doing the same with Chromecast is more tricky since the protocol is more extensive and most clients verify the certificates of the casting device to be authorized by Google.

probably worth to consider having official receiver for UmbrelOS [0] and CasaOS [1] in their appstores for increased user adoptions

[0] https://umbrel.com/

[1] https://casaos.zimaspace.com/