stormycloud is the biggest, there's also acetone and purokishi that are both very reliable. there's a couple others that aren't as stable right now. i2p's about inter-network services that often work without any need of the clearnet, tor is a better option if you're looking to do clearnet things.

torrents don't get bridged through the (mainly 3!) outproxies in i2p. they're bridged by random people cross-seeding torrents through clients like qbittorrent or bigly bt which work in both i2p and clearnet. Or, you can download a torrent from clearnet normally and change the trackers to i2p only trackers, then add to i2psnark. In both cases using biglybt/qbittorrent you should be able to connect to peers from i2p and clearnet through the people that do this, functioning as your own outproxy in a way.

Anyone can easily bring over clearnet torrents into i2p, and they are more than welcome to do so!

So i'm not disagreeing with you about i2p needing more outproxies, more is better, but tor does this already (and arguably does it better since there's so many exit nodes) so i don't think the demand is as big. You're right that it's pretty similar running an i2p proxy. As far as i know it's a very similar process running tor exit nodes vs i2p outproxy, i think acetone is also a tor exit node (i might be wrong on that), purokishi routes some things through tor, and stormycloud until somewhat recently mainly focused on running a fleet of tor exit nodes.

The nature of i2p means that to get faster speeds these providers may be running many i2p routers to provide lots of tunnels and load balance them, so i think that aspect is more technically challenging but i've never done it at scale myself. I think you'd need a handful of well resourced/connected i2p routers to offer a consistently good outproxy service.