Rocket Stove Hot Tub Heater - Boiler or Tankless

Hi there friends -

Here's the question:

Do you think an old oil residential cast iron boiler or a newer tankless heat exchanger coupled to a rocket stove would work better to raise hot tub water from ambient to 110*?

I understand these units serve different functions. The tankless heater exchanger is made to immediately raise the temperature of a small amount of water 30-50* while the boiler is meant to raise a lot of already hot water 20-30*. In a sense, I need aspects of both of these systems, but which do you think would work better.

Both systems would be paired with a 750gal per hour pond master pump (not thermo-siphon)
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Background:

I built a ferrocement 500 gal tub and a 7" J-tube rocket stove with permalite-concete riser insulation. It feeds into an old 50 gal water heater tank which has a 10 gal compressor tank (not sealed - ie no pressure) inside with a 30 ft copper coil. This was based on a YouTube design, perhaps from a permies member though I don't recall.

The heater can get 10 gals of water scorching hot in 10 mins, but it's just not up to heating the whole tub. Therefore I am looking at re-purposing an old commercially designed heat exchanger. I understand the safety concerns with steam pressure etc. but so long as the pump is running, I don't see that as a real risk unless my rocket stove magically starts putting out another 100k BTUs.

I know this post was 8 years ago.... but I like your idea and am also interestex in making a ferrocement tub like you speak of.
Are you using it now? I was wondering if it held up good and was  worth doing?

Did you find a suitable commercial heat exchanger? I know  most people would prefer to bui!d with only dirt and sticks ( just jokin, I love this site and forums! ) but sometimes its hard to beat a commercially made thing like a heat exchanger If its well designed and super efficient.

Adios!