Are you going to post after IR pictures.

Yeah if I can get another night close to 64°F

This is a fantastic application for a heat pump. you need a lot of 'low grade' heat delivered at around 110ºF. your 1kW heater delivers about 3300 BTU/hr. a typical 500-600W heat pump/window shaker will deliver 7-8000BTU/hr. that's 240% the heat delivered for 60% the operating cost, not including the fact that when you reach your setpoint it'll shut off.

judging by your note above, you pay approxmately $0.10/kwh. 24hrs*31days*1kw*$0.10/kwh=$74.4 for 1kw electric heater
delivered BTU/month = 2,455,200
BTU/dollar = 33,000

24hrs*31days*0.5kw*$0.10/kwh=$37.2 for 500w heat pump
delivered btu/month = 5,952,000
BTU/dollar = 160,000

I thought about using my heat pump water heater but I have no idea if it's insides can take bromide levels like found in a hot tub.

the general wisdom seems to be to use a secondary exchanger, or a titanium hx for direct contact. otherwise how long they last probably depends on the metal and pool chemistry.

When you work on the filter see if you can add a section on the return where you could add a heat exchange.

About 7 years ago we installed a hillbilly hot tub a 6' diameter galvanized stock tank.
we wrapped the wall with about 1" of flexible foam insulation and placed the tub on 2" of styrofoam wrapped in a tarp. We started out with a propane hot water tank with snap switches running the circulation pump. we tried a babington burner using used veggie oil, and later added a barrel stove, put in a liner & insulated the space between and made a heat exchanger out of 1 1/2" galvanized pipe in the firebox for the last 4 years we now heat the tub with wood. The stove is lower than the tub, so thermosyphons and needs no pump. We do not like chemicals so had to change the water at least once a week in warm weather. Two years ago we build a sand filter in a 50 gallon barrel and use a float switch and a shurflow 12V pump powered by a solar panel. Now we have crystal clear sweet smelling water and seldom change the water When we first fill the tank after our Mexican winter we first pour a gallon of bleach into the filter then fill the system through the filter, the bleach is very diluted and after running through the heater loses its chlorine smell. The firewood we burn is obtained from a local mill which puts its unused log offcuts in a pile and allows locals to take.
I think we have a pretty good system, tho we will have to buy a new stock tank for next summer.
Cheers, this is my first post for a few years as we are super busy building a house.
Just poured the master bed rm floor over radiant pipes today.

You missed the boat by about 4 and a half years.

Yeah, did not notice the date until after I wrote it............ better late than never.

It's rain water collection now, since the pump died.