Did a season of freewheels. Pain to swap out, as you say, and not as reliable as cassette freehubs in my experience, even with White Industries freewheels.
I've been racing with freewheels. 18t on one side, 20t & 22t on the other. Only annoying thing is repositioning pads when jumping from one extreme to the other, only takes a couple of minutes though.
Look at bmx race hubs if you want splined freehubs, Profile Racing should have something in 120 OLD
Not many options for 120mm cassette hubs. Google suggests the SunXCD, but it's expensive.