I think glider can do this, with -strategy rr (Round Robin mode). I have not used it in this way myself, so you might need to experiment a little. Proxychains can also do this, but it doesn’t present a socks5 interface itself (it uses LD_PRELOAD, so it won’t work everywhere).

We have those on I2P already, see tracker2.postman.i2p for example.

You should not torrent over the tor network, but you can torrent over the I2P network. qBittorrent even has experimental I2P support built in.

I’m not on NixOS, but I have a decent working knowledge of Tor.

Not quite clear on what you’re trying to do, are you trying to run a relay, or just connecting to the Tor network and pointing your browser to the socks proxy?

Arti (the official Tor implementation in Rust) is not a complete replacement for the Tor C implementation yet. Hidden service support is disabled by default (due to the lack of a security feature that could allow guard discovery attacks), and bridges don’t work either. If you don’t understand Tor very well stick with the old router.