I have enabled ubus over HTTP (what an interesting API, I'd definitely sponsor a more normal REST API!)
However, it doesn't seem to log anywhere. rpcd doesn't seem to have any calls to logging functions at all.
Is there a way to get rpcd/ubus to log incoming calls and responses?
A "normal REST API" is not possible because I can't represent everything as a resource. The only disadvantage of JSON-RPC is that you have to encode a file content to base64.
You can use ubus monitor to see everything what happens
You could absolutely have a REST API covering the main capabilities of OpenWrt and this would make it much easier to write integrations.