Those are large charges of fffg powder. More typically ffg in those ranges would probably be good. Sounds like you got it sorted out though. As was stated earlier, 75 grains can be quite good.
I had an old muzzleloader with blued barrel that I tested 777 in when it came out. Intentionally didn't clean it for 3 months after shooting. It actually wiped out with no damage done. That was left in a dry environment and your results may vary. But since yours likes being dirty with it, I'd probably feel safe cleaning at the end of your hunt.
Glad it's all working out for you. Differentbways to foul a barrel. Like you I fire a cap then load. I do that every time with a clean barrel. I do the same with all my modern rifles, not one of my rifles likes a clean barrel and I never hunt with a clean barrel
Shooting a fouled barrel is best on the range. But to be honest, not sure it really matters at ml distance. Unless it’s really long range. I’ve kept ML loaded, un fouled, for almost a year. Real black powder And the bore conditioned with “wonder lube”. Using hot RWS caps... never had a misfire and the bores never corroded.