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First dabble with flat mounts so hoping I can just check my thinking.
Rear. 34mm apart, 36.5mm from the drop out. Small adapter used to fit GRX400 caliper with 160mm rotor.
If I get a Hope Rear flat mount RX4+ +20 version, I can do away with the adapter and mount direct for 160mm rotor?
Front. 70mm apart 23.5mm from drop out. Currently a GRX400 with a 160mm rotor and one of those floppy adaptors which is 140/160 (or 160/180 as printed in it but I'm sure my fork mounts put me in the 140/160 bracket).
Could I keep that adapter in the 160mm position it's in now and add a Hope RX4+ rear +20mm caliper to run 180mm rotors?
Watching this thread with interest as I think I need to do similar
In theory, yes, it will fit, but to be sure all aligns, I'd probably buy a single rear first and then a second one once you're sure it does.  Does your fork allow for 180mm? You'll be creating a substantial bending moment at the new brake pinch point which will be in an area it wasn't designed for.
I'm working on the theory that if the tyre traction is the limiting factor, the purpose of the larger rotor is less effort required at the lever and marginally better cooling. It's increasing the torque arm by 12.5%. If that exceeds the design safety factor of the fork, I imagine I'll die regardless of the rotor size.
I like your idea of buying them one at a time though.
It's not so much about the change in force as it is about the application point of that force. One would assume that the fork leg is reinforced around the braking zone and then tapers that reinforcement to just normal loading once out of that zone. If you move the application point and you're now on the end of that taper zone, and you've increased the force a little, it could be the pair of factors which cause the snap. As for the tyre, it's always the brake>fork that take the initial spike of deceleration, then the tyre slides, so it will still see a peak force beforehand.
Also, will the 180mm disc clear the inside leg or the fork? Some taper inward outside the normal disk zone.
The forks will clear a 50mm tyre so the lower legs are pretty much parallel. It's a gravel fork rather than road, so I'd expect a bit more beef in them.
I'm not sure I follow your point about the application point. I'm not saying you're wrong, just that I'm not following it.
My thinking is that with the plus 20 caliper on the existing adaptor, the load is still acting towards the fork leg through the adaptor via it's two points of contact, those being the two threaded holes 70mm apart.