I've used https://www.elephantsql.com/ for smaller hobby like things with much success. It's not really "like Planetscale" though.
Thanks for this but 20mb of data seems too low. Even supabase offers 500mb
Yeah, you'll get more generous allowances from platforms that want to up/cross-sell you. If you don't mind a bit of extra fiddling, you could use fly.io. Very generous free tier there. Postgres isn't "managed" in the usual way though but it works.