oof, even for things like DO/Queues?

From the documentation for .json() we can see that it expects a arbitrary serde serializable type. This indicates that what it's doing is creating JSON text from the given value. So, you're sending a JSON-quoted string containing JSON to the server, which is presumably not what it's expecting.
I haven't worked with reqwest, but it looks like for your use case with a JSON string, you would use .body() to provide the string and .header() to specify the JSON Content-Type.

Negative lookups indicating that the key does not exist are also cached, so the same delay exists noticing a value is created as when a value is changed.

At the Cloudflare global network location at which changes are made, these changes are usually immediately visible. However, this is not guaranteed and therefore it is not advised to rely on this behaviour. In other global network locations changes may take up to 60 seconds or more to be visible as their cached versions of the data time-out.

if I read the key again with cachettl of 30 days would the key come back as null until the 30 days has passed? or would the write be reflected within 60 seconds or so?