The first two boxes that I would buy in lead is the Federal 3-1 5/16-#6 and same shell in #5s with the flite control wad; third is the Winchester 3-1 1/4-#6. You cannot buy Win in #6 in the box of 10 so it forces you to get a box of 25.Fourth choice is the Remington Nitro Mag #6s again in a 25 round box.
I have seen very nice 30 yard patterns from a 870 youth 21" bbl ,Truglo GSE,.565, and #5 shot from Winchester. You really just have to try them out.
At 30 yards using the Federal FC #6 and the Rem SF you should get a suitable pattern however.