i would recommend a good solid strip below the plywood along all of the seams if possible.
I would install blocking at all of the seams so that you can nail the plywood on all the edges. That way you know that the seams will stay lined up with no chance of warping.
I built a 28X36 hunting/recreation log cabin on piers about 15 years ago. I cut trees off the property and had a local guy bring in his portable mill and cut 6X7 logs. I didn't put any insulation between the joists. My reasoning was it's just a hunting cabin, and it will serve my purposes. It's mostly carpeted so the floor isn't uncomfortable. It has a wood stove that keeps it pretty toasty even in cold weather. It has served my purposes quite well, but that said, I wish I had put insulation in. I wasn't on TBN back then. Wish I had been and had asked lots of questions of you experts. I was an inexperienced builder, but all in all, it has held up well.
The best thing about TBN is somebody will come up with something that you never thought about doing, or even considered. I can't count how many times I've started a project, thought I knew what I was going to do, then realize that a comment on here will change everything!!!!
I just skimmed this thread, so not sure if someone has already mentioned this. If you go onto YouTube and check out "Boss of the Swamp" you will hear him swear by multiple times about a product for this exact purpose.
Shop Reflectix 2-sq ft Reflective Roll Insulation (24-in W x 1-ft L) at Lowes.com