Your resin shed with a floor will do great over pavers on gravel. For best results, make the paver base as flat as possible, to result in a shed build that will not be out of square or forced together. Get a few extra pavers to build a sort of ramp up into the shed.

Holy shit that's crazy bro.

Foil-faced insulation, I think, is completely vapor impermeable. If so, then you have to be ultra careful where you use it. Last thing you want to do is trap moisture inside your walls and end up with 50 tons of moldy stinky concrete wall filler.

In my climate zone vapor barriers are not really needed, so I avoid using them.

In cold zones you want the vapor barrier towards the warm in winter side of the wall, so that the steam from your hot cocoa does not condense into water droplets inside your wall.

If condesation is a concern then use the Rockwool product -- I forget the name.

Gosh, fucking concrete in the walls. Unbelievable.

Man, if you're brave enough to get rid of that shit... I wonder if closed-cell spray foam would replace some of that stiffness you are afraid of losing. (That would also be a vapor barrier, though).

If the studs are in good shape then I think the concrete isn't really doing much. There's that gap at the top of the walls. There's nothing sitting on top of the concrete, so I don't think it's really holding up anything.

I guess I'm just getting stuck on the idea of having such terrible wall insulation for the sake of keeping such a bunch of garbage inside the walls where the insulation should be.

Lego rainbow waterfall

Agree with Tkajohn that you need to go down to bare dirt.

Use redwood stakes to secure your perimeter foundation "sleepers" (the 4x4's) in place. You can sister one of those stakes across the butt joint. Or just use a flat nail plate or tie plate. I'm not quite sure if the stakes or a nail plate (or any other hardware) would properly counteract the outward pressure of the gravel over time (like how a cardboard box full of wet sand wants to bow out the sides of the box). I suggest adding another 4x4 across the floor plan of the shed at that point, then use one bigass tie plate at the three-way joint where they all meet (like if there were two monkeys pushing your sleeper butt joints apart, you can just tie their tails together to cancel their forces). Clear as mud, right? Um, okay, have you seen those picnic baskets that Yogi bear likes and they have two lids that hinge in the middle? I am suggesting that your sleepers are like the rim of the picanic basket, and that you should add the piece that goes across the middle (where the lid hinges connect) to tie across the long sides.

Save yourself HOURS of fucking around to level your sleepers by purchasing 6 bags of construction sand. Dig a 5" wide trench (with your dirty pickaxe) where the sleepers will sit, then fill with an inch or more of sand. Smoothly smoothy-smooth the sand to make a level bed for your sleepers. As you level the bed, and you hit some dirt that you didn't quite get out of the way, then use your thinking brain to resist the urge to level that dirt. Scrape that crap out of the way immediately with the claw side of your dirty hammer. Take out a little extra. Then just put in some more of that sweet sweet construction sand. Smooooooooth it out like a dream baby.

Before you even start digging, you can temporarily build or place your sleepers where they will go. Use your new four foot level* to find your low spot. That's where you must dig the least. The high spot is where you must dig the most. Yes, this is highly technical shit here. Hang on to your girdle cuz now we're gonna get real crazy:

Move your temporarily connected rectangle shaped sleeper agglomeration about 6" diagonally sideways away from where it actually belongs. Use a bunch of old stakes, plywood scraps, used Earth, Wind, and Fire 8 tracks, and whatever other flat garbage you have around to Shim Your Foundation to be Truly Level. Also, use your new FATMAX 16' tape measure to measure across the diagonals and make sure your sleeper building footprint shape is Truly Square. To paint a picture, your shed foundation will be sort of floating above the ground, sort of right next to where it needs to be. Congratulations! You can tell exactly where to dig-- it's right next to those floaty boards! You can tell how deep to dig-- measure from the top of your foundation down to the bottom of your hole the same distance all around. You can tell how high the top of your sand should be-- again, it will be the same measurement down from the top of sleeper all around.

Ok, that's cool that you can measure down to the sand, if you want to, but listen man... check this out, what ifffffff you screeeeed the sand? Have you ever heard of that shit, man? That's when you cut some boards with your milwaukee brushless cordless 6.25" circ saw-- and, like screw those fucken boards together all crafty like --- this might be like 3 fucken boards or something-- so that its like a measuring device to measure down to to the top of the sand, and then you slidey slide that device to actually smooth the top of the sand to the specific measurement you want. That is screeding. Screed your sand all around until the perfect flat top of your sand bed matches the perfect flat top of your sleepers.

If you have enough helpers then you can just all lift that foundation off it's shims and lay it onto its sand bed. Recheck diagaonals. Stake into place. Fill with gravel. Compact (you can do a passable job of compaction with just a frikkin 4x4 post. Don't buy that piece of shit 10x10 tamper with the wooden handle). Lay pressure treated plywood over compacted gravel. Use more nail plates / tie plates to fasten all the plywood together.

Then build your shed.

*your hardware store has a bunch of levels for sale. Some of them are accurate! An accurate level will read the same when you flip it over, or when you flip it end for end. If you check five levels for accuracy you will probably find 3 that are accurate for level. Then check the two bubbles for vertical! It takes 15 minutes to find a good level.

If you don't stand your 4' level against the wall then it can't fall down. Lay your beautiful blue Empire level flat on the ground after you use it.

Taller structure may also fit two yaks if it needs to

Get yourself some Simpson 3" SDS screws and a cordless impact driver. Works great for those flat "face nail" connections.