This would work but the recessed nuts are available in many different lengths; if one short enough cannot be found the nut can be cut down enough to allow the bolt to thread in sufficiently. IIRC the hole in the back of the fork will need to be enlarged to 8mm. Rule of thumb for minimum thread engagement is equal to the diameter of the bolt, so an M6x1 bolt would need 6 threads engaged; your 5 threads is a little shy of the mark. If you cut a nut short leave at least 6mm of thread, 6 full threads.
Edit: Here is a link for the nuts: https://www.ebay.com/itm/BDW-brake-c...s/232862309847

A little off-topic, but for rear brakes, drilling out only the front part of the rear brake bridge to adapt to recessed nutted brakes is difficult, here is a 3D-printed solution which allows drilling out both sides and adapting back down to the correct diameter: https://www.thingiverse.com/thing:3662440