Epically boring post, but added in case it benefits anyone else with a similar problem.....
A while back I had to replace the front TRP Spyre caliper on my Spesh Diverge with a mechanical Shimano caliper as it was the only the dealer had in stock after the Spyre broke a few days before an event. The power was decent on the Shimano unit but it always had a problem with howling and nothing would sort it inc thoroughly decontaminating the rotors, new pads and checking the caliper was centered. I was about to order a new Spyre and consign the Shimano to the spares bin, but thought I'd try swapping my front and rear calipers over.... problem solved and both units in their new homes work perfectly with zero howling, go figure!
The only thing I can think is that the Shimano only has one piston which pushes the pad and rotor across into the other pad and I wonder if for some reason that was the issue on the front where there is more flex than on the rear.
When switching them over I also greased the mounting bolts as they looked very dry and I doubt any grease was used by whoever installed them.
Incidently the Shimano unit is also far better than the Spyre on the rear where the Spyre always lacked bite.
As promised, epically boring but might help someone and spare them hours of brake fettling!