rqlite will give you HA, but won't help with load (rqlite replicates for reliability, not for write performance). But I like to think the it'll give you solid -- and simple to use -- HA. Replicated just replaced their use of Postgres with rqlite for this reason[2].
[1] https://github.com/rqlite/rqlite
[2] https://www.replicated.com/blog/app-manager-with-rqlite/
> turn it into an actual business :-) It'd be a lot more fun
Not only could it _not_ be more fun, but it might actually suck all the existing fun out of it completely. But the OP will know better about that!
As for your hypothesis: I can only speak for myself, but I just don't want to ruin my hobby by monetizing it. Plus, I've spent enough time with the community to know that as soon as money gets involved, people are suddenly not so kind anymore. I just like it more when people are kind and polite to each other and treat each other with respect...
I guarantee for nothing, since this is a spare time project. That said, I have had zero outages in 11 months, see https://ntfy.statuspage.io/
When it outgrows the single EC2 instance, I'll likely start building some distributed thingy to spread the load and do some HA things (sounds like fun). Probably just a load balancer and 2-3 instances with a rqlite backend instead of SQLite, nothing too fancy.
Right now that's not necessary. As you can see, it handles traffic just fine for now.