Welcome to the T&TH forum. I've been "away" for a while so pardon my not welcoming you sooner. You have a fine gun in that Remington 870. I shoot several of those in both 12 gauge and 20 gauge and find them to be great shooters.
I use both the thumbhole stock and the ShurShot stocks and see advantages to both. I like the longer LOP of the thumbhole but like the flexibility of the ShurShot. I'd hate of have to choose one over the other.
There are in my experience, several very good choices for shotshells and turkey chokes for your gun. In my Remingtons, I get great results with 3 different 12 gauge shotshells. In no particular order, the 3" Hevi-13 "Bronze" shell with 2 oz. of #6 shot; the 3" Winchester Xtended Range Hi-Density shell with 1.75 oz. of #6 shot and the 3" Winchester HV shell with 1.75 oz. of #6 lead shot all work well.
Any of those shells with the SSX Turkey Choke will deliver very dense, turkey killing patterns at 40+ yards when I "deep clean" my barrel. That's an important step toward maximizing the performance of any shotshell/choke combination.
"Cleaning a Shotgun Barrel"
outlines the "deep cleaning" procedure.
Best of luck and if I can be of help, just let me know.