I ended up completing this project, and was extremely satisfied with the result so I thought I'd share.
I used plywood for the board material for its strength and aesthetic. 3/4" seemed an appropriate depth for screws to get a good bite EXCEPT often, the tip of a screw doesn't have much to do with holding and it would be taking up part of that 3/4". I don't want to waste the back 1/4 inch of whatever material I use.
I used lathe boards that were 1/4" or maybe 5/16" thick to create a gap behind the plywood. I used trim nails to attach the lathboards around the perimeter of the plywood, and across its midline for support.
The concrete surface was not perfectly even, so I decided to use 'great stuff' on top of the lathe boards and on the plywood inside their framing to create an air tight seal so bugs can never set up a nest in the void I created.
To physically attach the plywood to the concrete, I balked at tapcons. I'm putting up a lot of board and tapcons are tedius. They're also slow and with great-stuff in play I have to work fast. I went with a borrowed Ramset, and I have to say it worked Pretty darn good. I used 2 1/2" ramset nails that came with a washer.
One yellow cartridge was enough to start them, Usually a green cartridge or two was needed to get the nail all the way in.
WARNING: DO NOT use a yellow cartridge for 'followup' shots, as it can more easily overdue it and blow the nail clear thrugh the wood! I would have been happy to find brown cartridges as they're even lower power than green, but after much searching, I think they're a myth…
I attached temporary handles to the plywood to make lifting easier, and i intalled temporary hooks to the ceiling to help hold the heavy plywood up and near position straight and a few inches out from the concrete. This was important because all greatstuff has to be applied before any point on the panel is fixed to the concrete. I didn't want to smear the great stuff all over the concrete by attaching one corner and then raising/rotating it to attach another.
Levels were used to make sure each panel was straight. Chalk line was used before putting any panel up to create a clear line on the front of the panels for all nails to follow and a neat line. Making the chalk line over the center latheboard is essential before the board is up on the wall as that latheboard is not visible once mounted.
WARNING: When attaching two adjacent panels side by side, do NOT install a ramset nail in the corner of both of them. Leave a good 8-12 inches in the concrete between each nail to manage the 'stress' you create in the concrete with each shot. If you nail every corner, then the adjacent corner nails will be only 2-3 inches apart and it will cause unnecessary damage to the concrete.
WARNING: DO NOT use ramset at any angle other than 90 degrees. It will wildly blow out chunks of concrete.
Instead, to get around obstructions like large vents, ignore the perimeter lathe board, and add another horozontal piece that is in an area not obstructed.
All ramset nails should/must go through plywood AND latheboard to prevent creating a depression in the finished surface.
It's also a good idea to start by scraping or hammering the concrete surface as flat as you can before you begin if there are imperfections (seams) that stick up from the surface.
Wear eye protection for sure when using Ramset and hammering concrete. Ear pro is not a bad idea either. Gloves and paint suit are wise to prevent clothing damage from greatstuff.
Moisture was not a concern as the concrete wall is one of two interior sides of a garage slab, so there is little opportunity for moisture to get behind this wall and it's never happened since the house was built.