Nice - .065" rifling. I didn't know Ed was making "Forsyth"-type barrels, but then, he is capable of cutting any twist, any depth.
The powder charges seem in order for the rate of twist. You might find for plinking, that 120 shoots OK, to 100yards, but longer range shooting needs more to group.

I agree, the thicker patch will be better. The shallow rifling might allow a .610" pure ball and your .018" patch for even better accuracy. The .600" might be good in a WW ball with 12ounce denim. 12 ounce is .025" as my mic, makes it, and .030" average in the 2 sets of dial calipers I have.

In my .69, I find a .675" WW ball and the 12 ounce shoots virtually the same as a .684" and a 10 ounce patch. Our bore/ball sizes ie: your .600" vs. my .675" have similar ratios to the bore size.

New rifles are fun - lots of testing to do. Have fun and make her work.