Hey well I should pipe up because I actually plan on doing just this -- though I planned on giving back, essentially doing a profit/revenue share and projects sponsorship assuming it actually got off the ground. My project hasn't launched yet so I don't mind getting feedback, I'm planning on calling it "Ragtime Cloud" (it is really hard to think of names that fit in well with joplin without mistaking myself for the brand itself).
Up until now what I wanted to do was offer WebDAV based syncing services (there are varying problems with the other methods except for DropBox as far as I have heard). Since Joplin Server does sync much more quickly, it makes sense for me to run it as well and offer it to users (even in it's experimental state). There are more features I want to offer based on Joplin -- basically, I think Joplin could rival Notion but it needs innovation on the hosting side, as the app is already pretty fantastic, thanks to your hard work.
More to your point, I'd say that the AGPL will not protect your interest in this way, what you should go with is the BSSL (MariaDB, Sentry, other decent companies have gone this route). In general, the AGPL will not prevent people (me, let's say) from using your code without contributing, they just require that you contribute back if you modify it. So if you run the code unmodified, technically you don't have to contribute. I have no problem contributing (and was hoping to, but have been stuck in a bunch of yak shaves along the way which is why Ragtime Cloud still isn't even up) -- but just want you to know how it stands as I see YOur other points on AGPL are correct -- most people who run it locally won't care (if they even understand the licensing to start with).
If you're interested in talking about this feel free to send me a DM on here and we can chat/email or get on a call. The part of my skillset that is relevant is infrastructure and I think the Joplin community needs a known-compatible sync/backups provider (and from there growing into more things).
@laurent tagging for visibility