They are not being as used as I expected/hoped.

Have you considered it’s because of this?:

My only requirement: these instances should remain closed for registrations and used only to create communities.

I wouldn’t run an instance that didn’t allow users to sign up as it would impede growth and uptake.

It also would have the interesting effect of pushing a lot of the load onto other instances, which doesn’t seem true to the Fediverse spirit.