Well then, the simplest solution would be to interrupt the power to the heating element. A relay/contactor controlled by an ESP board would do the trick. Or even an off-the-shelf smart plug. Insert an outlet and a plug in the circuit to the heating element and just plug the smart device in between.
I’ve looked at controlling my spa via HA, and I’m seeing all the roadblocks @neel-m mentions. If and when I go down that path, I think my approach would be to buy an ESP-controlled multi-relay board, capable of handling the currents involved, and take the existing, proprietary control panel out of the loop completely.
The problem is there’s then a useless control panel, or a hole where it used to be, in the spa. The shape and size don’t match any existing water-resistant screen I’ve seen. And controlling the spa through HA, while using the spa, would probably require voice control. Personally I’d prefer old-fashioned buttons or a screen, but they’d have to be totally waterproof. Remember electricity and water don’t play well together with human bodies.