It likely doesn’t matter, anything from 6 to 20 will work if you go to failure. And 2-3 sets is right. You don’t even have to worry about progressive overload since you are taking care of that with the first 2 exercises, just create a lot of local fatigue in the target muscle. Go with a weight/reps that allow you to get the best mix of mind-muscle connection and load.
Personally I like to do 8-10 for the isolated assistance. Lighter than that I get less of a mind-muscle connection, it feels like the muscles are not contracting as hard even if I’m squeezing and more weight than that I lose the mind-muscle connection again because I compensate with other muscles or slight position changes. But everybody is different.
How do you feel about the maximum overload training principles where everything is done in the 4 to 6 rep range?
Probably good for the compound movements but not so good for the isolation movements for the reasons you stated earlier about mind muscle connection?
Right, you know, if someone can get a good mind muscle connection doing heavy sets of 4 and 6 on isolation work they can definitely go for it. I personally can do that on pecs, quads, traps and hams. But not for biceps, triceps, delts and lats. But it is likely an individual thing.