I have a custom steel road frame (Shand) with post mounts. In the time it went from order to arrival, the world of road groupsets shifted to flat mount!
My solution was simply to run new Ultegra R8020 shifters with “old” RS785 post mount calipers. I was advised by my LBS that they wouldn’t be compatible, but some deeper research showed that the hoses are compatible and I’ve had no issues in nearly 18 months and 3,000 miles of use.
So – to be clear – you can use newer post-mount compatible Shimano shifters with post mount calipers. Even if you can’t find older post mount Shimano Road calipers, I’d be fairly confident that you could match all their road offerings with post mount MTB hydraulic calipers so long as you check and match the correct hoses.
This will mean that you need to buy shifter and caliper separately, but I don’t think that’s a major problem.