I'd like to second the recommendation of ACP230. If ever there was a powder that can be used in almost any application, UNIQUE is that powder. RedDot/Promo is an excellent powder, which burns cleaner than Unique, but lacks slower burning speed oft needed for +P loads in .45 Colt or magnum-level loads in shotguns. If most of your shotgun reloading is for skeet&trap or dove &quail, then the heavier loads that're often made from Unique won't really matter, and RedDot/ClayDot/Promo might be the way to travel. I've used Alliant Herco when I couldn't obtain Unique, and it has a slight edge in +P .45 Colt. I can't say how it works in shotguns. It was originally marketed for use in magnum shotshells, so I suspect it'd work well. You mention that you're not trying for heavier loads, but 00 buckshot and slugs tend to be pushed rather hard. One can use Unique for lighter loads in 12 gauge, and increase the charge if heavier loads are needed. I don't know that this flexibility is as evident with RedDot.
I'm not as familiar with the Hodgdon/IMR/Winchester/"who's next?" line of pistol/shotgun powders as I probably should be. An old shotgunner friend of mine said he'd go out of his way to use IMR PB and nothing else in his .20 gauge, for all power levels. I've used it in.38 Special, .45 Colt and .45 ACP, and like it very well. It will bring out the full power of the .45 ACP without excessive pressures. It's a bit too fast-burning for heavier (more powerful than factory) loads in .45 Colt, but very clean burning in every application I've tried. Were I in your situation, my first choice would be Alliant Unique, followed by Alliant Herco, followed by IMR PB.
I hope you find this information of help.
