I've noticed a lot of games allow a mode selection whether it be easy, medium, hard or different play styles in a diplo. What could be done instead is when available, making the computer slot user selectable. Then determining the game mode based on if a Nexus, CC, or Hatchery spawns.

I don't remember if UMS allows computers to be user selectable, and I don't recall if user selectable will spawn a town hall/4 workers. If not, most efficient would be to use an EPD to read the race in memory. User start location could be random, and each player can choose a race corresponding to the difficulty they play, then whoever is the last player chosen on the force gets to set the difficulty.

The issue is that race is not intuitive, and the simple civilian > beacon method is tried and true.

You can use any sort of build trigger: Zealot is easy, Dragoon is normal, HT is hard, DT is expert. Make 1 scarab/interceptor for every level of difficulty you want. The problem is that these options aren't intuitive.

Most intuitive methods I have found is lifting a structure or multiple beacons. There have been a few with random stuff like unloading units, killing a unit... But these tend to take up more needed resources. Hence using terran structures because the lift command makes them less desired to be used otherwise. Beacons are just really simple to label.

It's a good idea so I just tested it. "User Selectable" sets them to "Random" but you cannot change it, and they do spawn buildings.

You can also consider the number of locations being consumed. If you use killing as a selection system, you don't need any locations.

Assuming you select difficult at the beginning of the game and then never again, you can simply use another location for the purposes of difficulty selection. Obviously not a location that you want to be permanently tethered in place on the map, but if you've got a location that frequently moves around the map (centering on various units at various times), then you could re-use it for difficulty selection.

Contrary to what I said before, I just tested this again in 1.18.whatever. You can select CPU race now in UMS, so is indeed a viable method.

That sounds like an awesome bug/feature.