Selecting the right on-demand water heater for my DIY Jacuzzi in tub

[droidus]

I have a regular sized tub. I would like to be able to heat the water to a certain temperature. I've been looking at on-demand heaters for this small project. Not sure if that's the best way to do this. I'm also open to other methods of accomplishing this. If I do add an inline heater, I'm also going to have a water pump plugged in.
First off, I don't feel comfortable taking apart the control panel. I noticed a lot of these water heaters require a certain gauge of wire, and it seems you have to go through the control panel, which I don't feel comfortable doing. Not sure if there is any other way to figure this out.
Some of the products I've been looking at:


Source: https://www.amazon.com/Bosch-Electric-Mini-Tank-Heater-2-5-Gallon/dp/B0148O658Y

Source: https://www.amazon.com/Ecosmart-POU-3-5-Electric-Tankless/dp/B0047RAQZG


Source: https://www.amazon.com/Ecosmart-ECO-MINI-2-5-2-5-Gallon/dp/B00M05ISBQ
Would these be too small/under powered? My bath tub is about 59" long, 18" high, and 27" wide.

One thing I noticed is that the breaker is 240V; what device in the bathroom would be using 240V? Could the fan in the ceiling be using that much?
What are my options? I'm open to other possibilities as well.

I've attached photos of my control panel. I believe it is #8.