With the help of Claude I created an alternative
https://www.stremio.com/ app, for every platform supported by Rust and Flutter (not tested on all, not built on iOS) that shares torrents back to the universe, uses almost-official stremio-core to handle existing Stremio accounts and add-ons, and also provides new features (so far only "dl for offline viewing").
It took a couple of weeks (mostly spent patching a fork of upstream torrent library rqbit) and for the most finicky bits (cache invalidation...) Fable was almost necessary, but I haven't even looked at the code so far.