Communities is definitely limited. You can’t sort them, you can’t filter them (particularly by instance when looking at All).
The best way to discover new comms seems to be using Lemmyverse, apparently you can set it to your own instance so that links come up in your instance, where you’re logged in and can subscribe.
Sites to Discover Lemmy/Fediverse Communities, Instances and Stats
Lemmyverse - Filter and find Instances and Communities...