Revisited: Spa (Hot tub) Renovation - Gas?

First, thanks to all who responded to the previous thread. Highly educational.

Here, I add a little data, and correct a few bloopers.

I've just inherited a working 8' wide spa (AKA hot tub), fiberglass body in a 2" spray-on-foam-insulated wooden frame with an insulated lid, holding about 500 gallons. It's an Apollo Spa # AP 96960, with a pack that's clearly OEM'd from elsewhere, a CM101 28-Z132NVVXS-AP, serial BA97101-010. Apollo is up in Pugetopolis, and that's where my nephew bought in when he moved to Seattle, eight years ago.

We're going to install this outside on concrete, on what was a small (non-reg.) basketball court. A windbreak or polyfilmed gazebo will surround it.

My county's building code requires a separate circuit to feed the 40A

240V draw of its control panel (AKA 'pack'), dual-stage pump and electrical heater. Installing that circuit, and the new panel it would require (as the existing panel can't handle it) will cost $2,400, as per two estimates from reliable electricians.

A hot tub manufacturer's website estimates that at 45 degrees north it will cost $380/year a) to run the pump on low by timer to keep it clean, b) to run the pump on high when we're in it, and c) to heat it to the temp most folks like. That assumes our electrical rate is at its current Enronesque 8.4 cents per KWH.

Has anyone here reworked a similar hot tub for gas heat?

One issue raised by Apollo when I calle dthem was the greater longevity of electrical heater elements and lesser replacement cost, as opposed to gas heat exchangers. Any comments? Bear in mind the electric elements have already eight years on them.

Natural gas and LP gas are much less expensive to heat with here (our electric utility is owned by Enron, happy, happy, joy, joy), and if I can replumb it to use an on-demand heater,

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36 KBTU heater would cost less than $ 800. It also would reduce the size of the electrical install; I wouldn't need a separate circuit and therefore would not need a new electrical panel.

A 150 gal. propane tank install would cost $220, with an annual tank lease of $70. Refiils of propane now cost $1.63/gal., and 4 gals of propane (assuming 68% efficiency) will bring that 500 gal tub up from

55 degrees F (groundwater) to 105 degrees F.

I am trying to find if the heater aforementiond can work with natural gas. Rheem has several (AKA RayPac) for natural gas for about the same price, but do not have the thermosiphon capability, and automatic operation. Here's a 50 KBTU model:

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It would be nice to use the existing control system ('pack') but those can be had for