If you want more control over your hot tub than can be done with more standard approaches, here are some ideas of what others have done. These are geared to electronics hobbyists.
If you are instead looking for more standard solutions, see Wi-Fi control of a Softub.
Arduino-based solutions
Here’s an example of someone using a custom Arduino to replace the existing controller board. It integrates with the Softub using the existing temperature sensor and display:
CircuitPython-based daughterboard
I made a custom daughterboard that fits between the control panel and the temperature probe. It’s a powerful solution, but isn’t something you can buy off the shelf.
It is an open-source project that an electronics hobbyist could make. It also uses Home Assistant and gives you lots of control in configuring your tub and controller.
For example, I had things configured so that holding down the lights button on my Softub controller turned off the lights in my house. It also reports the temperature to Home Assistant. View the project on GitHub.