I do use Group DMs quite a bit and it's kind of a
visibility issue for me to try and determine what
Group DM is what based on just "3 members"
without clicking into each one.

I think it's a separate issue from the ability to
bookmark them, which I do see a request
for already.

Perhaps something like "(3) user1, user2, user3"
could work. I understand setting a custom name
for them is a workaround, but it'd be nice not to
have to do that for each one.

Yeah, I'd like to add that. It already does it correctly for Slack, but it's a bit trickier for Discord (I hadn't kept it in mind when designing the system for Discord.)