I’m not sure if this was mentioned before, but a few months ago I was working on a similar proposal and hoped we could move on from using RLP to either CBOR or SSZ.
If we want to do that, perhaps we should not wrap payload into RLP, instead we could choose to use a single byte as transactionType. Could also consider this field as “version”. This allows for 256 different types, which seems okay. This would also mean that signing with transactionType = 0 would be easier, than with the proposed extra RLP wrapping.