Honestly this is what I use most of the time because it's hit the mark nearly 99% of the time. Quite a variety of recipies, and actually the comment section is genuinely full of useful information!

t2bot.io uses webhook/bot based bridging which at least doesn't trigger the "no custom clients" clause (as you aren't using any real user account tokens)

The broader question of whether bridges are allowed seems to be broadly yes as there are many other bridges out there for Discord (IRC/slack ones) and those haven't been shut down either.

I think so long as you aren't abusive, Discord don't care.

riot.im hosts its own Jitsi instance which is built into the UI, so hitting the call button in a room will spawn an embedded Jitsi call in the room.

Recently we added the option for people to use their own self hosted Jitsi instances, if they prefer.

(1:1 calls at the moment are normal WebRTC, directly over Matrix)

I might be breaking the rules, but can you point me towards any information about how to integrate my existing self-hosted jitsi server into my self-hosted matrix install? Finding this seems elusive.

I tried and i don't think its easily possible.

Thanks for the clarification!