Earlier quoted context omitted.
The TailScale folks caught a bunch of flak for their "do things that don't scale" approach to databases. But, honestly, most startups would be better off following that approach than what Fast did. Sounds like the engineers were just entertaining themselves with shiny toys rather than solving the problems they actually had.
Tailscale's "database from 2022" is neither here nor there. What you should do in this situation is use the most boring tech, such as PostgreSQL with a simple HA setup. Thus avoiding the "play with nifty toys" trap and maximizing the chance of making appropriate choices.
I've done something similar before and it saved countless hours wasted on configuring databases, writing queries, fighting impedance mismatches. Postgres brings a lot of work. With a file you just need a couple of lines to write your memory out to a file and a couple more to read it back. Simple and effective.
Until it stops scaling, but at that point you've established that your business is successful enough that it warrants the effort of doing something shiny. Postgres has a place in this world, but no need to put the cart before the horse.