torrents didn’t have official support till fairly recently and it’s still a little wonky

I don't think this is true at all. They've both had solid torrent support for years, across multiple major version numbers. It's neither wonky nor recent.

I’d say you’re probably going to want some custom scripts. Have Radarr move the file and rename as normal and then your script to symlink it back to the torrent directory under the original filename so it can continue to seed without taking up double space for every movie

Further driving home that this dude is full of shit, hardlinking the files is enabled by default in both Sonarr and Radarr and certainly doesn't require any custom scripting.

OP, quit listening to random people online and spend some time reading the documentation yourself.