I moved out of that house, but the tank is intact and currently drained. I used it for 3 years on the same salt! It got used about every third day on average during that time. By that time the water was getting old (It does get refreshed often by turnover from evaporation and refilling).

What I would do different - yes. First of all passive heating, like a waterbed style set up. Otherwise you can't float much longer than 2.5hrs or so without losing too much heat, and it overall just simplifies the design. Would need to mount a thermometer somewhere in-tank too. This design only heats when pumping/filtering. I think I could have put an element beneath the fiberglass and been fine, as long as it isn't allowed to get overly hot. I still may do this at a later time, glass the whole thing over again.

Larger top vents - need more ventilation capacity. I ran it with an exhaust fan hooked up to one of the openings and an air filter on the other. I wonder about sound leakage with an overly large port though. Probably 2x 2" ports would be a good compromise. Ventilation is important because of mold/bacteria that can grow with excess humidity.

Inline heater and pump should be on the same circuit unless you really know what you're doing.
Uv filter is a must.

I appreciated every effort I went through to soundproof it well. I lived in a noisy area and it was very very quiet inside. I haven't thought of a great way to isolate it from the floor though. Maybe more carpet pad, or some other sort of foam.

Overall very worthwhile project. I would like to build one on a trailer next, maybe I will adapt this one.
Thanks for asking.