Joist design for shed/workshop
I recently build a 8x14 shed using 2x8 pressure treated joists and there was no bounce.  It felt solid with a 3/4" PT plywood floor.  Your design is larger, so I agree with the 2x10 that you spec'ed.  I don't think you'll have an issue it all.

I was originally going to try and save some money so I mocked up a few combinations of 16" oc and 24" oc with 3/4" and 1/2" plywood since it was "just a shed".  The joists 16" oc with 3/4" ply was the only combo that felt solid (as expected).  The shed had a small 4' "porch".  For that I did use 24" oc joists with 5/4" PT deck boards.  That was a stretch that I wouldn't have made for a full on deck, but it was fine for the small porch that would only have a few people on it with no static load.

I had 6 piers, so your 8 sound good.  The only thing I second guessed is the depth of my piers.  I was digging alone, and manually so I didn't get them as deep as they should have been.  I didn't notice a problem, but I would suggest getting the recommend depth in your area for sure.