As I noted within my post #9955859@lemm.ee (alternate link), I found that thumbnail generation in Element is an enourmous privacy, and security vulnerability. Thumbnails are generated server-side, regardless of E2EE settings. What this means is that the URLs that one sends would be leaked out of your encrypted chats to the server.
Indeed, it does. It can be overlooked, however. I added that info to my post, though. Thank you for the note.
With my friend we have a bot who does this for us. Check out on maubot - at least it is mot client side work