I think community discovery can (and should) be improved for sure!
Currently it's true that you can use topic-centered instances for this, I do this myself as well, but I do think it has quite significant downsides in terms of creating pockets of centralization. For example, if you're a user who is ONLY interested in french cinema (or any specific topic) on Lemmy, and all of the related communities and other invested users are on a single instance, then for you, the experience is absolutely no different from any centralized platform - the french cinema instance admins have 100% control over your Lemmy experience.