I feel that, while lemmy is still a work in progress, it is already pretty adequate for solving this need. If you want to subscribe to other instances you can do it from within your insance by going up to communities and searching. You can also click the all tab and see a bunch of instances from around lemmy that your instance is federated with.

I think mastadon struggled with this because the twitter model is to follow people and depending how far removed the servers are this can be trickier. Compared to lemmy where people interested in a single subject will likely target and find the subject theyre interested in and bring themselves together naturally.

Furthermore I think some people are splitting up and dividing into sub instances and tiny subjects a little prematurely. Reddit didnt get super esoteric with it's subs until it got big and the larger subs either declined or got too noisy to talk about certain things. Like for example how beehaw has an operatingsystems instance instead of a linux, ubuntu, macos, windows, fedora, archinux, opensuse, openbsd, etc. Right now there arent enough of us that we dont need to subdivide.