Going through the same thing with the powder supply. 8208 is the in my option the Grendel powder, BUT I have not seen it available in well over a year. 8208 is not temp stable and your pressure will spike when you get to the max end. 27.7 is my stopping point for 123 ELDM in my 20” barrel, I feel more comfortable at 27.4 which give me 2530ish FPS.
CFE is good and accurate and I have a couple of pounds on the way from Brownells. I can get better violocities with CFE223 but, not temp stable if you are concerned.
I have done 1 ladder test so far with the ar comp and it shows promise. I started at 26.8 at 2477fps and worked up from there. I would recommend staring a little lower with your staring charge and work up from there. 2530ish seems to by my accuracy node for the 123gr bullets. The SST shoot REALLY well out of mine as well.
If you find any of those 3 powders grab them and shoot!