I guess I'll go with option 4.
I don't know if any radical change is needed. I run a small MC server for my extended family (about 15 users on the whitelist), and I've noticed it goes through regular waves of dormancy where nobody will log in for a few months, followed by periods of pretty intense activity where several people will be playing about every other day.
So I think those cycles are probably normal. And we are just in one of the quiet phases right now.
Maybe one thing that could be done would be to find an interesting new area where the community could set up a new town or some other communal build project. Could also increase the number of res slots per player to encourage people to build and explore.
Please don't shut the server down forever! I've played on a lot of MC servers. And this is one of the best ones I've ever played on!
Finally, thanks to all of the mod team and server admins for all the time and effort you put into maintaining this fantastic server!