I have a different style gravity feed, so take it with a grain of salt. I never shoot past my set temp. Not sure exactly why you do that. Maybe you get too big a fire going in the fire chamber and then it burns dirty on the cool down. I let my pit come up to what ever temp I plan on cooking at and let it settle in. I like cooking big meats like butts and briskets around 270 to 280, so that is the target temp. The controller I use may over shoot the temp a little but only 5-10 degrees at most. It then settles in and doesn’t fluctuate much at all once settled. At that stage, I put the meat on. It burns clean for me.
As for charcoal, I don’t really care for Royal Oak Briqs. They seem to leave more ash and a slightly bitter flavor. The best thing about Royal Oak or knock off brand made by Royal Oak is that they are readily available and about the cheapest price, at least around me. Never used Cowboy lump in anything but my kettle. It’s ok, but piece size is really varied with two many small pieces, that make me not want to put it in my Gravity smoker. Kingsford standard always has its own flavor to me. Not the best, or worst, just taste like Kingsford to me. I like others better. My two favorites are Jealous Devil briqs, and Kingsford Professional. Jealous Devil are large and dense. Clean burnings, and probably my favorite overall. Kingsford professional are much lighter, and not as dense. They burn really clean though and maybe leave the least amount of ash. At least that is how it seems to me. Because they are lighter in weight, they burn a bit quicker. That is my only knock. Costco tends to have them at decent prices in two packs which I stock up on. Easier for me to find regularly than the Jealous Devil. I would definitely try some other charcoal to see if it helps.
What kind of smoker did you use before the gravity?