As @Masimatutu points out, for a community on another instance, you normally only see the subscriber count of people from your own instance who are subscribed there, which can make other instances look smaller than they are.

If Lemmy could have some inbuilt support for tags or subscribing to a topic / multi-instance community,

I think you are right. Kbin now has multi-communities called Collections, and it's a bit of a gamechanger.

We can make our own and/or follow other people's public multis on various topics. I am now seeing so much more content from way more instances.

You can see global active users and local subscribers, which is enough for me, since people on my instace are more likely to have the same interests. If I need global stats, though, there is https://browse.feddit.de/

Remote access of other communities is a smoldering mess. You have to hope the community you want to subscribe to shows up in All or you have to know its name and use the search feature. There's no way to see another instance's communities, unless you have an account there, which is a huge problem because you have to go through the huge hassle of signing up for an instance just so you can find the community, and oh yeah, most instances aren't even open to invitation so it's not even possible in a lot of cases.

Also if you get banned from a community it force-unsubscribes you, and if you get banned from an instance it won't even tell you.