hmm, is this roughly equivalent to Neon[1], but sqlite based rather than postgres?
This is true. Neon however offers bottomless storage and D1 is 100Mb currently going to 1Gb.
Neon doesn't have read replicas and even if they already work on it, I wouldn't expect it before 2025 if at all and never at CF's pricing (Neon still charges for egress).
[1] I would compare D1 rather to Turso or LiteFS from Fly or PlanetScale with many read-replicas