Drainage is important, and you know that. I would use a water shield as an outer barrier. 2" poly isocyanulate inside that against 4" cinder/concrete block. I used 1/2" radiant tubing inside that and former the inside with mortar. These were mud bath tubs at a spa. 30yrs later they are still going. I am not sure what you would want to use for an internal finish. It would be nice to have a smooth finish. Seems it wouldn't matter in a mud tub, but would in a bathtub. Also, I guess it being outside you will need a cover as well as a cap for the walls.

I did a lot of research on this since I had been so unhappy with the tub we had in our previous house. We had the typical jetted whirlpool tub. Even with weekly cycling bleach through the piping, we would still get black flecks pumped into a clean filled bath when the jets got turned on. If you Google "black flecks/flakes/gunk whirlpool" you'll get lots of info on this. When I looked into the air tubs, it seemed that there was still about 1/2 cup of water that would sit in the piping and could mold etc. I found out about Sanijet tubs and looked at their website and spoke to a couple of very helpful people when I called Sanijet. They have a pipeless system where individual jets intake water and the jets are easily removable and can be cleaned in a dishwasher. (View videos of this on the website) Another huge plus is that these jets are much less noisy than the typical whirlpool tub. Ours is ordered but not yet installed. We got a big 2 person tub. I think it measured 72 x 42. They run specials periodically. We got ours at 50% off list price. The company has no showrooms. Fergusons used to carry them but I read that Kohler (which is their main brand) made them discontinue that line since Sanijet states how hygienic Sanijet tubs are which I guess infers that the other tubs aren't. I hope to be able to report back here once I get into the new house. The company will tell you the nearest Bed and Breakfast or hotel that has their tubs installed so you can try them!

Honestly I would call a local pool builder. You are basically wanting a indoor spa. 12 on 12 #3 rebar, with 4 bars at the top and a little thicker 12", and 6 to 8" of concrete/shotcrete in the walls. You can insulate under it, but remember that is a lot of weight, and insulation could crush over time causing it to move. Not really necessary. Running heat strips in the pour sounds like a reasonable idea. Unless water table is high or something don't see sealing exterior. Seal interior well and probably tile it, just like you would do any wet location. Again, call a PB, they could hook you up.

You need to actually need to go sit in one I love to bathe but in noway can I see that being comfortable. I also do not think a cast iron would be very easy to carry and install. IMO a nice deep soaking tub not free standing is a much better choice. Have you actually tried getting in and out of a free standing tub most are really awkward. This definately is a thing to test drive and you will need all the plumbing to be visible so expensive too.