Treated lumber won't rot for decades. Or regular 2x4s sitting on something like cement blocks, bricks, short pieces of scrap 2x4s.
Don't know how much you know about framing but could frame it up like a wall and lay it flat then level it up good then cover with at least 1/2 OSB or whatever you can find cheap. Are you using the whole garage area? That'll cost a pretty penny.
I have the same kind of carport here that I've been thinking of just walling off the back 10' and basically building a 10x15 room inside the carport. The carport is 24x34' with this crappy red shale that runs up the driveway to the highway in front. Gets everywhere! I've had the place for 16 years.
What are your winters like and is the garage heated? I'll be using 2x6 for construction if I ever do it but probably won't. Get down to -35C here in the winter and +35 in the summer in northern Alberta. I have enough room in my cool basement to add an 8x4' grow space to my existing flowering room but took over an upstairs bedroom for vegging plants last fall. The basement is really just a concrete box under the house for the plumbing and stuff for our dugout water. Stupid setup IMO as if there is ever a serious leak it's lower than the dugout and I'll end up with a 10x24' below ground swimming pool. Already had 8" of water in down there a few years back when there was a drainage blockage down the road from us during the fast spring melt. Fast melt this spring too with more snow than usual but no water in the basement since they fixed the problem. Still had to get the province to send out a loader to clear the culverts or might have had another flood. No cost to me tho.