School me on weird brake wear.

How snug was the pad to bracket interface? I've had pads come in the past with backing plates seemed to be slightly oversized and that caused them to bind up in the brackets and cause strange and premature wear. My solution has been to remove some backing plate material with a bench grinder and dremel to make sure that the pad is free floating in the bracket. If the mounting ear on the pad is sticking in the bracket that could help to explain what you're seeing.

Since you had new hardware on one side I's likely not a slide pin to bracket issue, but that's the other big one for uneven wear. The slide pin and mating hole need to be periodically cleaned out and re-greased with good high temp disc brake lube. When it gets old/dried out/overheated it tends to bind up.

The taper I ASSumed you were referring to was *in my head* back to front rather then top to bottom, I suspect something about your stuff has spread/stretched and is all junk
Time to disassemble and compare calipers/brackets to units that have never seen a racetrack.

If I a wrong, no clue...

Are the inner and outer pads worn the same taper/direction ? If so, in my mind, that kind of cancels out the sliders.... Crappy pads or overheating will burn away quickly but evenly.

I agree with the flexing thinking. My best guess is the "fingers" opposite the piston(s) are flexing when squeezing. They also have the disadvantage of only reaching 60%? of the way across the backing plate to make things worse.
You could try to take measurements while someone stands on the brake pedal if you can find somewhere to consistently measure across the caliper opening.

What was your pedal feel like compared to with the ST43s?  More pressure required?

Do you get a lot of pad knock-pack?  Meaning a long brake pedal going into a corner following a high-G turn (or hitting a curb at track-out)?  If so, that's "spindle flex" and/or loose wheel bearings, and can do weird things to the pads.  And your underwear if it is bad enough.

What does the pad on the other side of the rotor look like?

As far as pad fit into the caliper guide slots goes, they should rattle.  The pad will get hotter (and hot faster) than the caliper, so thermal expansion will quickly eat clearance at that interface, and sticking pads are bad for a lot of reasons.

Maybe the calipers spread over time.  It is a thing.

Hopefully the other pad in the pair will help with diagnosis.

Could be the additional pressure required was flexing the calipers more.  Could be if the backing plates were tight they were binding when hot and latching "on", and if the guide tabs are nearer the outside edge of the pad you were getting more pressure on that edge.  I dunno.

This might be one of those "replace all the components and go back to decent pads" situations and hope the problem doesn't reappear.

Didn't you have another weird braking issue recently with this car?