I think the best way forward might be to deprecate all rpcs that use json encoding for message transfer, specify and implement order-preserving cbor-encoding for legacy messages, and provide new rpcs that use the cbor encoding. A few months after those have been rolled out, all the rpcs that send json-encoded messages can be removed.

If this issue forces us to deprecate and eventually delete those messages anyways, there's no real benefit in replacing them with order-preserving json, we can directly go to the cbor subset.

If we go for cbor-encoded messages, we can also do a compact encoding of the metadata rather than putting them into a self-describing format. So this involves some more design work than just doing a 1-to-1 conversion between json and cbor. But this is what I would have been working on soon anyways. I'd simply postpone the clean-up work (and completely drop the json-encoded metadata if we remove them anyways), and specify and implement the full cbor message handling instead.