Gagguino is a great example of this approach, licensing scandal aside†. Espresso machines are expensive and not because the software is particularly clever. They are electrically simple, but mechanically there's a lot of pressurized plumbing that you really don't want to DIY.

The kit is a control board for the pump and boiler, and some add-on sensors for temperature and pressure. The "high end" features that it enables are almost entirely software driven, the main one being temperature control via PID. I've seen even simpler mods for other machines that bypass the "brew" button so you can do things like connect a bluetooth scale to enable brew-by-weight on a machine that doesn't support it, or add a shot timer.

The commercial version of this would be the Decent, but it costs 3x as much. I would love something like this for my robot vacuums. Valetudo is minimally invasive, but there's no reason you couldn't control the vacuum + wheels, but navigation is hard and those sensors are much more complex (can you even access the camera and undistort the image?)

https://gaggiuino.github.io/#/

† they pulled the rug on open firmware