A small go service writing to a sqlite db would be pretty easy (and probably pretty performant) but needs streaming replication to improve robustness/durability. For that see:

https://litestream.io/ and https://mtlynch.io/litestream/ (an example) Edit: I just re-read the mylynch.io post and it probably has all the code you need.

Probably could run it on the fly.io free tier + S3/B2 storage costs.