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Best check the Shimano compatibility chart but IIRC you can use the latest road shifters with the latest MTB postmount calipers and they all use a BH90 hose. The older road 785 postmount calipers use BH59 hoses yet people seem to have been mixing them with newer shifters without any issue.
If you buy a full shifter set (with hoses and flatmount caliper) and intend to use a MTB caliper then not only will you need to sell the calipers but there's a good chance the hoses won't be compatible too - you'll likely need a hose with a banjo connector on if you buy MTB calipers.
You may find it cheaper to buy the parts individually but few shops online are selling each bit individually.
I recently went through it all. I bought GRX shifters, SLX calipers, and the correct hoses and connectors all from German shops (bikediscount, bike24).
Tychom-
hose with a banjo connector
Yeah good shout, I realised this earlier. All the MTB calipers use a banjo connection. From memory my old 785 calipers didn't have a banjo connection. So to avoid buying new hoses too, I'd ideally get a second hand pair of 785 calipers.
This is just for a winter hack bike, in an ideal world I'd find someone selling the older RS685-RS785 shifter/caliper combo. But it could take a while waiting for some to show up.
TotalShanner
The current gen 105 r7020 hydro groupset doesn't have a post mount option it seems.
Say I buy the groupset with flat mount calipers, I can simply swap them out for older post mount calipers? There's no weird compatibility issue or anything?
I'm pretty sure I can do it since I done the reverse with red etap, went from post mount to flat mount.
Just need to be sure there's no issue. Any advice appreciated ta