I’m migrating because Transmission is horrible for a large amount of torrents (multiple of hundreds)
That doesn’t sound like too many, you’re saying you’re at under 1000 torrents? How many multiples of hundreds are we talking?
Surprised Transmission has issues seeding that many, thought Transmission 4.x made improvements in that area. How much RAM does your system have? Maybe at some point you just need more system resources to handle the load.
PS - For what it’s worth you can still stick with Transmission and/or other torrent clients & just spread the torrents among multiple torrent client instances. e.g. run multiple Transmission instances with each seeding 1000 or whatever amount of torrents works for you.
That should be easy for just about any torrent client (including Transmission), could be worth opening an issue at their GitHub page https://github.com/transmission/transmission/issues
Hopefully switching torrent clients resolves that for you.