I’ll go to bat for Roborock. All common “user serviceable” parts are available for direct order from them and remain the same between generations so even very old models can be easily maintained with first-party parts.

They unfortunately won’t sell you an internal part like a motor, but you can still find new first-party parts if you know where to look and it generally only takes removing ~10 screws to get at the insides (example).

The low end Roomba is simple to repair. There are lots of YouTube videos you can watch about disassembly and gearbox cleaning. The battery is easy to replace as well. The only bad thing about its design is it’s made from plastic. In eight years or so you might need to replace the brush cage or a similar part. Plastic loses its flexibility and gets brittle over time, so some parts like this might split or crack.

My personal preference would be a consumer model, <$400 usd, that doesn’t require wifi or bluetooth for any functions (preferably not even having that capability)

We have a Roomba I’ve replaced the main wheels on. They seem to have a good availability of spare parts as well, although they are in big units. I haven’t found a way to replade the tracks of the main wheels, as the ‘tires’ are the issue here, not the rest of the unit. They wear fast with uneven flooring.

Ive been told the Makita DRC200Z is very user servicable, mostly because its a more commercial space dumb unit.