You'd be better off getting some post mount brakes with IS adapters, Shimano MTB brakes are fine with GRX levers but get deore or something with a straight connection, SLX and above have banjo fittings so you'd need new hoses too.
Thanks for this. Sorry, but what's the difference in connections between straight and banjo? I've got hoses but not fitted them yet so could always get other hoses if better brakes would be better
Straight goes directly into the caliper like on all the road/gravel brakes, banjo has a fitting at the end that goes into the the caliper at a rotatable 90° angle, often on MTBs there's more stuff to fuck about with and it's beneficial to be able to adjust where the hose is coming from.
You'd be better off getting some post mount brakes with IS adapters, Shimano MTB brakes are fine with GRX levers but get deore or something with a straight connection, SLX and above have banjo fittings so you'd need new hoses too.