Thank you! Everything you say is accurate and matches up with my experience. The connectors and stuff you can purchase as a regular consumer barely offer the ability to read or clear diagnostic codes and swapping almost any part requires a specialized connector or thing to connect to. For example, doing even the most basic sensor replacement or heaven forbid a ECM swap. I am thinking of scenarios where Grandma calls and has a Kia where a basic sensor is malfunctioning. You'll be paying out the nose to do Kia authorized things that only Kia will let you do, and the change of that being under $1000 is virtually zero.
A KDS subscription is $30 / 3 days.
https://kiatechinfo.snapon.com/J2534DiagnosticsAndProgrammin...
They claim that only an expensive J2534 interface is "recommended" (a weasely way to get around compliance requirements, although J2534 is also a terrible standard and frequently not compatible) but based on what I've read, the Kia software (KDS) is really simple and even cheap J2534 cables like a GD101 ($30) will work fine.
At this point you have the same software as the dealer would have, so any sensor related issues should be solved (besides ADAS, which like I mentioned is definitely a problem due to the need for calibration jigs). You could swap in a new ECM this way. Not even a dealer can swap a used ECM on some Kias; although in this case there are reverse engineered reflashing tools that work in most cases, too (this is kind of an intentional gap in right to repair, especially in Europe - there's a strong drive to eliminate the used car control modules market because it is heavily associated with organized crime).
I really don't find auto diagnostics to be as sinister as people think they are, or the regulation a complete failure. You can, due in large part to regulation, wade into the morass of horrible dealership diagnostics software if you want, for a limited entry fee.