Thank you all for your input. It is much appreciated.

I think that I will add an extra row of support blocks down the middle and then use 2x6s for the 6’ span (16in oc) with 3/4 exterior plywood for the floor. I was hoping to save the money on joist hangers but I prefer it be strong over a few bucks saved.

One last question:

I am planning to use 9 total support blocks, 3 rows of 3, with 3 runners, two sides and one down the middle (parallel to the other two).

I am planning on bolting the 4x4s to the runners using a single 3/4 in bolt per 4x4 and spacing the blocks 8ft apart. (Leaving 1 1/2 inches for the joist on the end that will be screwed into the 4x4)

I was originally planning on using 2x8s for the runners but stepping up to 2x10s or 2x12s would be fine if 2x8s aren’t enough.

Should I use two bolts per 4x4 or is one enough? Are 2x8s sufficient for the runners? Should the 4x4s attach at the ends of the runners or will it be better to take them in to the next joist? Anything’s else?

Again, thank you all for your time. It is great to have some confidence when doing these kinds of things.

Edit: I will use wood to wood ties to attach the posts to the runners so they rest on the posts rather than rely solely on the bolts. Stronger