It’s lined with cedar and not heated or dehumidified. So I’m going to heat and dehumidify it. I’m also going to remove the cedar and put 1” foil faced insulation board on, cover it in an air/vapor barrier then add furring strips and finish it off. Add 2 nanawalls and a window. Some spray foam here and there too.
Sounds like you need a good commercial HVAC company who has designed and installed systems in indoor pools in your area. Ask around with other buildings, commercial or residential, that have indoor pools.
The rest should just need a good building contractor.