Finally finished my in-depth experimentation on toroid baluns last night. Turns out that the simplest solution is often the best: a traditional transformer acting as a voltage balun did the trick!
I was a bit dismayed with the narrow bandwidth of my system with the copper pipe dipole with a 4:3 step-down ratio (it hit the center of 6m, but the SWR at the FT8 frequency wasn't as good), but but changing to 6:3 (5:3 didn't work at all) brought the resonant frequency down and got one of the best SWR readings I've seen. 👍
My plan is to have a switch on the balun to select between the 4:3 and 6:3.
like this