The normal suggestion is to go with the 6 inch 3/8 grade 8 bolts. They are sort of the standard and sort of a known quantity. I used them for years on private. I mostly did presets with them and an electric drill. I've been watching the videos John has been putting out and I have some private permission about an hour north of me that I plan to thoroughly scout post season and prep for next season. I got the OK from the owner to do whatever up there, so I have been giving the setups that John uses a lot of thought lately. It's 280 acres of thick stuff with Ag all around, so that is a lot of room for some sets. The reason people say the 8's will bend is that couple of more inches gives you a lot more leverage. Is it enough to bend one, not sure?
In the videos, he states he uses spike a lot, and the ones he is using are not grade 8, just steel spikes. He does say that as long as you are 165 pounds or less then they are OK. I'm 155 pounds soaking wet so this is the route I may go if I satisfy myself that they are safe for me. He said he has turned some heavier guys onto spikes, but they cannot use 3/8 and have to use a 1/2 drill bit and 1/2 Rebar. For his presets he is hammering them in with a hatchet/hammer so these are permanent.
I plan to pick up a few steel spikes at the hardware store and try them out at ground level. Can't endorse it personally but If John has been doing it all this time there has to be something to it.
Get a couple of grade 8 bolts 8 inches long, put them in a tree about 6 inches off the ground and jump up and down on them and see if they will bend. You didn't mention how much you weighed. Just be careful.
I sewed up a bolt bandoleer for 3/8 bolts that can hang off a belt that I plan to try out. John uses a 2 pouch carpenters style belt for preparing presets. In one of his recent videos he gives the phone number of the guy who makes the preset pouches for him so people can call and have one made.