At this moment, Matrix is fairly different from discord, specially the pieces around voice chat there are some plugins and other things, but it is nowhere close to Discord's experience).
I have my own Matrix server and I use it to join any other that I need. By rolling my own, I can also leverage some integrations like having my Google chat, signal, and a few others in a single app.
In a way, because you are not looking for a "local feed" like in mastodon or even Lemmy to a degree, it doesn't matter that much which server you join, any of them will give you the capability of talking to others (sans the integrations I mentioned), kind of like e-mail.
It is very different from the feeling you have with Lemmy, I would say. The only way I have to describe it is as a Modern IRC.