My recommendations would be something like this: (I'm just a random user, so it's just my point of view)

• Shut down the fully inactive instances. Noone will even even notice it
• Merge the semi active communities to a handful of instances, like sports and technology... . I've seen active communities move instances, it would be possible, take a look how !europe@feddit.de migrated to !europe@feddit.org. Give enough time for subscribers to notice and subscribe to the new one.
• Allow registration of moderators on these instances, so they can work around the current limitations of moderation tools. Maybe an invite only solution or something like this.
• You could find help more easily if you look for admins for 3-4 instances instead of for 18 instances.

This would be useful for you and other admins, because you would have to admin much less number of instances. They would be still considered small instances, compared to big one, so you still not at the "too big to fail" level. For users it would help community discovery, there are overlap between followers of similar topics, e.g. I have friends who follow both European football and NBA at the same time, I read both selfhosting related topics and about general tech support, etc...