Thanks for reminding me about TiDB. Turns out I was on the first thread here about it. The next had some interesting comments:

https://news.ycombinator.com/item?id=13298664

Particularly, the comments about them mixing Go and Rust wondering if they should move to gRPC. The integration complexity concerns me a bit but at least they're smart about what pieces they use. There was also a lot of detail in the architecture although consistency and availability vs Spanner was not clear in my links. I'll have to look into it more. Thanks.