Have you considered a UX similar to old reddit for this? where you hover over the join button with the mouse and it gives you the option to create a feed or add and remove to a feed?

I actually checked multiple times and could not find it (but admittedly maybe that is my mistake and not a real problem with discoverability).

Active users is the standard metric used to check how much a service is used (at least as far as i know. its what i see when i look at stuff published for investors).

hexbar is on the sixth place in term of number of active users with 1.8K , lemmy.world is 18K (enable the “active users” column and sort by it to see the full list)