Reading through your post here and realized who you are... It's nice to talk to you I'm a fan of your channel, and I refer a lot of people to your videos when they ask for information on the "clones" as it were... That being said, I am sure you don't need my advice, but to address the "wandering" idle. I built one a while back and noticed a similar problem. I swapped out the carb and the problem existed still. So I dug into it deeper and found that it truly matters how the intake boot is oriented on the cylinder and how the carb is mounted on the boot. There is enough play at both points that if it is out of place it will cause the throttle cable to be tighter than it should be.. I adjusted it with care for the orientation at both points and the problem went away. I am certainly not telling you that this is your issue, but it is a possibility and something that can be easily checked.
Long sense forgot about pride. But after the time with developing the 660's I'm not quite as willing to do the R&D anymore. Those little aluminum brackets for 372 have changed from when I first used them...to the point they no longer allow full throttle on OEM builds. BUT might be their answer to the wandering idle on 372 style saws if that was where you intended that to go. AND those aluminum brackets work well on the X-torq to OE conversions where you use the XT carb...Also on some of the 372 AM cylinders the intake casting itself is a bit too long and that mouses up the geometry relative to the carburetor as well. Of course a better solution is OEM throttle cable or trigger parts. Especially on the g372xp & xt. Me? I just swapped in an OEM handle on the g372xt.....all kinds of things got better. On the Stihl clones, I did find the carb to carb boot intersection issues as well and rather than fight with the parts just swapped to "used" OEM part by part from the junk pile until the saws ran well. The g366 was the worst / 361 kit as well. SO they got parked. I don't have the time. Some one else's turn.