@Jennifer Darby if it's at least 5 mm thick, it helps with unevenness.

More of a concern for you would probably be the weight of concrete counters in the 2nd story units, as they really need to be at least 3 in thick to avoid cracking.Butcher blocks will scorch, stain and are pretty easy to scratch/gouge.  

In the back was the chef that grilled the steak to whatever thickness you wanted.

R31 min in the attic- our mild zone, now they suggest insulate below the floor, wall being that thick as well for those with crawl space. 

What I get out if it, is I build it once and it will never fall down.The home stores sell these thick metal bars, but they are more expensive.

The steel isn't *that* thick, but you need a high-test hole saw to install a lock in one if there isn't a hole already; a previous owner at my rental didn't understand this.On my rental, I used the interchangeable cylinder locks from Landlord Locks. 

No carpet unless the ground is uneven like a garage conversion or addition with poor foundation, if carpet get a thick pad.

If you are in a multifamily situation, it may be best to do LVP with a thick rug on top, LVP/hardwood makes a lot of noise.

Plus, because it's twice as thick, I suspect the click and lock more forgiving of the flaws in the subfloor.

A buy and hold investor probably needs a lot of patience and a thick (bullet proof?)