I have only ever used Winchester AA hulls for reloading 20 ga. target & field loads, only in 7/8 oz.(lead) loads ... and have never loaded buckshot.
Of the powders I have used, WSF , Unique, Green Dot & 800X, I preferred the Winchester SuperField (WSF) as it was easiest for me to get hold of on a regular basis and provided very good ptaterns for Skeet & Sporting Clays.
Alliant's Green Dot seemed to give the densest patterns, particularly with Winchester AA wads. The clone wads, like Claybuster, seemed to pattern just not quite as well, but were certainly serviceable for target & hunting loads.
You can find lots of load data available online from the various powder companies like Hodgdon & Alliant, and Lyman prints a pretty good Shotshell Loading Manual with a ton of different loads in it.
For large shot sizes, like BB & Buckshot, I think you would really have to go to a 3" shell, ith different wads & perhaps "fillers" like card wads, to adjust heights for a proper crimp.
Copper plated (actually copper washed) shot provides a little less bore scrub & pellet deformity contributing to better patterns, and some have found "buffering" materials like "GREX" interspersed within the shot to help tighten patterns ( Remington & Winchester both load "copper plated" & "buffered" lead ammunition ... mostly for heavy turkey loads.