Yes thank you that was what I was trying to remember. We really wanted to have the invite flow as part of the email we sent with other login details for other tooling, but we never got it working, not even with matrix's identity URL.
We were hosting through etke.cc, some issues may have been due to the specific decisions they made, however they were quite capable it seemed to me.
This was two years ago so the identity server was difficult to find, I think sydent may not have been as officially "canonical" back then or perhaps not quite so easy to set up? It could be on me but I recall it being a blocker I didn't have time to resolve after taking a crack at it.
I'm happy to hear you're working on things for element x however we recommended our members not to use the element x app since it didn't have the full featureset of element such as threading, which was critical to our usage (threads for gigs for example). Perhaps it has threads now though!
I support your project, I loved having the duplicators or whatever they're called mirroring slack messages and Instagram messages to matrix, that was part of our co-op's selling pitch for a while: "get access to a working matrix deployment running duplicators for Instagram, slack, some other things, so you can use these apps for messaging without having them installed!" I really wanted it to work but we had to choose the lame easy option with the lock in in the end. I am sure we will pay for it one day when discord enshittifies.