Looking good. Best to make a platform out of 3/4" plywood under the rubber mat.
You'll need an area in front of the rack to do dead lifts. The plywood under the rubber mat will protect the concrete, which you don't want to crack.
Most recommend 3 layers, so for 8'x8' that is 6 sheets screwed together.
The rubber is actually more important in front of the rack where you'll do deadlifts.
Garage gyms
Lots of advice in that thread also.
I had to violate my own advice because I'm tall and have a low ceiling. As it is I'm limited to 25 lb plates when I press.
I went with two sheets.
My setup is like this. I used 3/4" rubber pavers on the sides (where the dead lift sets down) .
I personally like my feet on the wood surface especially when squatting. So that strip in the middle is a good surface plywood, but underneath is the cheaper OSB.
Workout Area - Google Photos
Craigslist is your friend on plates. I like the old school deep dish Troy's and Hamptons.
Rustoleum is also your friend.
Of course I got all this before there the SS stuff was available. That probably would have changed some of my choices, but I like the way my floor came out.