FWIW, my Suntour GPX derailleurs played nicely with a set of Shimano L-422 Light Action friction shifters. I tried that setup for a month this summer, just for giggles, since I had the shifters in a box but couldn't remember why I bought them. I'm guessing my rationale was they'd make a good backup shifter in case the GPX shifters ever broke. Sure nuff, they worked very nicely. And the freewheel was less critical. Didn't matter whether I used the Suntour Alpha or SunRace freewheels. Even a too-wide 130 OLD rear wheel from a newer bike with cassette worked fine.

The GPX group has a reputation for being a bit finicky, which Suntour itself alluded to in its shop guide for setting up the group. But after trial and error I found the biggest improvement was simply switching to the SunRace freewheels I mentioned above, and KMC Z72 or comparable chain. After that all the persnickety minor adjustment problems I used to gripe about vanished.

Suntour itself could have preserved the reputation of that system by supplying freewheels with better shaped teeth, and better chains -- the original Suntour chain was awful, one of the most primitive, poorly made chains I've ever seen. I have to wonder how many people were put off the entire Accushift groups concept when the main problem was probably that terrible chain, and the Alpha freewheels (which could have been improved with a hand file or moto tool, for the daring home craftsman).