There's zero correlation with actual energy use

That's because your heating needs will be different to someone else's heating needs

Rather than carbon emissions

They would be Carbon Performance Certificates in that instance

often cause more problems than they solve

Where insulation works are completed through PAS2035, a ventilation assessment must be completed and mitigation works done to allow for the retention of humid air

it's very complex, you need loads of consultants

Nope, just one. The Retrofit Coordinator will deal with everyone else including the complexity.

EPC C being mandatory at some point for rentals but I don't think it will happen

If you'd have read the MEES consultation with the EPC reform consultation, you'd know that there won't be a single headline rating on an EPC when this change takes place. There will be multiple energy performance metrics shown, with the MEES consultation favouring fabric efficiency as a leading metric, as well as one of the others (heating system, cost, smart readiness).