Although it's technically in code to run pex under the slab, it's a very poor decision by the plumber to not use fosta just to save $. And that is a lot of water under the slab. Hopefully it's sleeved but looking at that work I doubt it. I recommend going with a tankless water heater as well. Being that it's plumbed for the garage it's probably pretty far from the master, where you could put the tankless anywhere.
I'd strongly suggest spray foaming for insulation. We did all our exterior walls, ceiling and under the metal roof. I think we paid $8k additional but it is so cost effective. Our heating and cooling bills have been next to nothing.
I'd also suggest tankless water heater, like someone else mentioned. We put ours in the attic and it's pretty quiet. It was additional $$ but it saves on energy costs.