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Leo,
I'm sure you know how high the tensile strength of steel is compared to most other materials. You haven't mentioned how big the load is but I assume it's a modest mount and OTA. Looks like you have 16mm rebar there.
The multi-layer inground pier is a great idea. i have thought about doing something like that even on a property I own so that when selling, I can pull out the top layer, throw in some soil and grass it over.
With the loads imposed by most modest amateur gear, 3/8" bolts are more than enough as is thin pathway grade mesh. Let's face it, you ain't mounting the Mt Palomar 200 inch.
Given that you want to make it removable and able to be hidden, less is more. Most people go way overboard with this stuff. The concrete does need to provide a secure anchor to the ground so that the moment of the mount and pier can't tilt the slab. But if your ground plate has say 8 anchor holes (3 per side) 1/4" anchors would be fine, if it has 4 holes in each corner, I reckon 3/8" would still be plenty. Some of the people offering advice, live in corrosive coastal climates. Lithgow is much drier and less corrosive on steel.
When I was 16 yo, I asked my ATM mentor if 1/4" thick steel plate was strong enough to hold the mount head to the top of the tripod. I'll always remember his reply. It went something like this, cleaned up for the forum, "expletive me Joe, expletive expletive expletive expletive expletive. You're not going to expletive pull through one expletive quarter expletive inch of expletive steel plate"
cheers
Joe
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