I vaguely remember some people experimenting with replacing the head unit with aftermarket ones, but no idea how well that would actually go in practice

This varies wildly from manufacturer to manufacturer and even year to year. For example, GM cars used to route damn near everything through the entertainment unit, so that was your central computer. Cell antenna, on star control panel, every that phoned home. That was as recently as mid 2010s. It also led to hilarious problems where a relatively simple issue like an OnStar button not working well required a complete replacement of the stereo unit (which was $8k or so in parts and labor). Now that instrument clusters are doing more while also getting more diagnostic and digital, things are transitioning to a more centralized computing system somewhere else. This can make it easier OR more difficult to get around, depends on design.

For other brands it’s borderline impossible to even use an aftermarket system. Mazdas for example the entire infotainment system relies on itself. There’s nowhere to even put a traditional aftermarket. I’m sure it’s possible, but the design of the interior is completely based around the infotainment unit.