Hi, I have a summary similar problem, broken right ST-RS685 shifter but with BR-RS785 brakes. I initially bought a ST-R8020 to replace it as couldn't find a new ST-RS685 and only more expensive 2nd hand well used ones. Then found out the ST-R8020 is BH90 rather than BH59 so I bought a new brake (BR-R8070). Then found out the fitting is flat rather than post so bought an adapter. Then found out there is not enough space on the frame to fit the adapter! Bike is a 2015 Cube cross-race. So options I can think of are:

1) buy a 2nd hand BR-RS685 (over £100 for a well used one) - at least I know it'll work
2) upgrade rotor from 160mm to 180mm - think this should work as the post to flat adapter is a lot smaller. Rotor is a centre lock one so am I likely to run into compatibility issues (I'm scarred from this!)? And probably need some more tools?
3) find another B59 compatible dual shifter - all I can find is a ST-RS505 but that's 2nd hand as well, half the price though of the 2nd hand ST-RS685.

All help appreciated.

It does seem that my frame is the problem - guess I could buy new forks? Or buy a new bike if I'm going to run into these compatibility issues going forward!