I've been in a lot of discord servers and a few things really stick out to me when it comes to finding the more fun communities to be in:
1. There's a sweet spot in membership where a server is populated enough to be interesting without getting overwhelming - The largest servers I'm in honestly get muted. I check in every now and then, but it's the smaller servers with a dedicated membership that I find most enjoyable.
2. The better servers tend to be the ones that vet their membership - Regardless of size, a lot of the better servers I've been in are the ones that either require you to be vetted ahead of time, or only give you access to a portion of the server until you've been active for x amount of time. No one likes an echo chamber, but it's important to have people that mesh well.
3. A server is only as good as it's moderation staff - I've seen servers (and forums) with mods that are too lax or too strict, but the best communities I've been in are the ones where the mods have clearly defined rules and guidelines, and won't hesitate to act when a user is breaking those guidelines, regardless of that user's status.
there's more of course, like community feedback, etc; but I always suggest looking into discords that are either smaller or that vet their audience in some way at least if you want to have a good experience, or - better yet - start your own smaller server with some friends to get used to it all. Once you've gotten used to the smaller communities, that's when I start to suggest checking out the larger servers like what tapas and webtoons have. It's a lot less overwhelming that way.