Quite a setup to host and maintain your own server. Matrix doesn't use MLS so it's quite inefficient and chat participants need to request keys for old messages constantly. The UX is just not very good because those messages seem to be "lost". But it is a decent solution for large scale sovereign group chats which need to be secure.
Discussion
Not sure if I answered your question
Yes. What do you think Nostr, especially Nostr DMs can learn from it?
I'm not really an expert on either so I'm not comfortable giving a decent answer to that but I guess the metadata problem in Nostr DM's doesn't exist on Matrix because matrix server traffic is not public. Matrix is too bulky for DMs in my opinion but it provides excellent privacy and unruggability. It's just not efficient enough in contrast with any alternatives using MLS (many will come I believe). Nostr on the other hand is pretty lightweight but has terrible privacy, similar to Bitcoin itself when used that way. I have no idea how to implement Nostr DMs "the right way".