I know you're not asking for this, but distributed consistency can get pretty hairy in anything but the simplest of cases. I know you've probably heard all this, but for the people unfamiliar with CAP theory, if you're asking yourself the same questions and you haven't spent much time researching it, it's worth it to get an over-view. The WikiPedia page is pretty simple, but has links to decent references:

https://en.wikipedia.org/wiki/CAP_theorem

And if you want to get your math on, check out TLA:

https://en.wikipedia.org/wiki/Temporal_logic_of_actions

(Again, the wikipedia page is laughably incomplete, but has references worth clicking on.)

A relational system with the guarantees usually provided by them seems really difficult to do. Like cockroach db but considerably harder or with considerably worse performance.

I always thought it was more common for stuff like KV stores with looser guarantees

You could probably run a CockroachDB edge cluster