> JSON is [...]

What would you suggest instead given all these cons?

Fixing all of those at once might be a bit too much to ask, but I have some quick suggestions. I'd say for a more robust JSON you could try Dhall. If you just want to exchange lumps of data between programs I'd use Protobuf. If you want simple and freeform I'd go with good old sexps.

https://github.com/dhall-lang/dhall-lang

https://protobuf.dev/

https://en.wikipedia.org/wiki/S-expression