The communities are still developing, but they'll likely always be more fractured than Reddit was. Right now users, especially lurkers, are congregating to the most active instances and communities. Sounds like those aren't the communities you're most involved with.
The other thing to keep in mind is that the user history and contributions that Reddit is built on generally don't exist here yet.
As you look around and get a feel for things I think you'll find it feels more active and engaging than your initial impression.
Agreed on all of the above, including buying premium just because it is good.
UI also has good customization options to fit whatever you reading habits or requirements are (font type, size, color, page turning, etc)