I think maybe the torque introduced by the aggressive riding is slightly flexing the fork legs and causing the rub. I had an old MTB with first-gen hydraulic disc and QR hubs... and the flexy Judy Suspension fork definitely contributed to brake rub, even though none existed in the truing stand. Not a great solution from a cost standpoint, but maybe swap the fork out for one with a thru-axle and get a compatible wheel?
Or maybe just use two credit cards inserted in the caliper when you gap the brake pads like I did with that MTB.