I can’t say too much about the other stuff, as I’ve only used the D’Brake (both external and internal).
However, to me the “Shimano 180mm Standoff mount” looks like a standard IS-to-PM adapter (International Standard to Post Mount). They make these in all kinds of sizes so you just have to get the right one. You do have to be careful as they are different for front rear (a 180 front is a 160 rear) and these days the IS-PM is more common on the rear of a bike (this is detailed in another thread somewhere).
If this is right, then you just buy the appropriate adapter for 160mm rotors (don’t know it, but the yours for the “TRP Spyre calipers” is likely the same, as almost all calipers take post mount). Also for lateral brake placement to match the rotor, you should be able to shim or grind the IS-to-PM mount a little bit (shim should be up to 5-6mm, grind probably only 1-2mm).
You’re probably aware of this already but the other difference between the UCM and D’Brake is how they mount. The UCM floats on both bearing cap bolts while the D’Brake is tightened hard against the rear bolt ear. You can see how the D’Brake is offset to allow for this in the pictures in this thread.
I’m not sure whether it makes a difference in practice, but the UCM might be slightly less forgiving of poor setup when it comes to vibration.
I now use UCM with Schlumpf, and d’Brake on the other unicycles. They both work great.
EDIT: I forgot about the adaptor thing. The UCM uses front spacing so if you want to use a 180 disk you need the 180 front or 160 rear adaptor, for a 160mm disk you would need the 160 front adaptor. I believe it’s the same with the d’Brake but I would have to double check.
Eric, thanks for the link. I’ve seen the thread but missed that post. Very much what I was looking for (I think).
FWIW, with D’brakes I’ve always just used the 160mm adapter supplied with my brake caliper/rotor. I only asked the question because MountainUNI specifically sells a Shimano 180mm adapter. Sounds like it’s a non-issue to keep doing what I’ve been doing.
Gonna let this marinate for a bit… Need to figure out what’s going to be easier.
160mm front adapter is quite rare I think, so I would expect that the 160mm adapters you got with brake is actually 160mm rear adapter which is exactly same as 180mm front adapter provided by MountainUNI.