I'd go with a tank too. Really efficient for your setup.Yeah I wish I had done it!!! I may rip the floor out and do it at some point.... Id probably do a tank instead of the hot water heater also. and recirc back to the water tank.
Under my floor wont work well. Its a 2x2 grid of tubing underneath. And I foamed the shit out of the bottom side too.
My thinking is laying 3/4" foam over the steel floor, routing for 3/8" tubing, and then laying a similar vinyl plank over the top. I would have to figure out door transitions, raise the fridge, and transition to under the kitchen area as I built it on top of the floor.....
What's inside the 2x2 grid of floor frame tubing underneath? I assume the grid is all hollow square tubing welding together. This is just me and my hair-brained ideas, but I'd explore the possibility of plumbing coolant through those tubes if the ends are accessible at all. Maybe they're laid out in such a way that you could manifold two groups of them together to make a feed and return. Might need to bore through the length of one perpendicular tube to manifold all tubes together to create the full loop. That's the best way to get heat transfer that I can think of right now. I would guess that your floor plate is welded to the tubes and you already have the tubes buried in foam.
Again, that's the hamster wheel in my head spinning off its axle. I have a tendency to make everything work double duty, if not triple duty. You might have the option of utilizing the structure that already exists. Hollow tubing is a favorite of mine. It takes up space, so might as well fill it with something.