Have you been reading this forum for the last few years? There are people who don't tune at all. They take what they are given and that's it.
Yes, there's no official designation. That only matters for Sport Mode, and that's fine. There's nothing stopping leagues and such from designating which specific cars can compete. In fact, that's a necessity for a balanced event. Not to mention the custom single player stuff.
As for the "hassle" it's actually elemental gameplay. Ubisoft has an instruction program to teach game design, and one of the elements of game play is "puzzle". Puzzle game play is the "you need to figure it out" part of a game. Tuning, tweaking, etc, is all game play. Sure, it's not for everyone and that's ok.
You quite literally described a race car.
Take the 911 RS CS. Add the widebody and all the racing parts and you get the 911 RSR. The car in the photo below exists in the GT7, but you have to go make it.
View attachment 1495454
And the BMW 3.0 CSL? The widebody version was never a road car. We have Miatas and Skylines that are designated as "different" because of a change in year and some other minor details, but the 2 versions of the CSL is only 1 car??
Then of course, the F40/F40LM that I mentioned. The XJ220/XJ220LM that I mentioned. They are street version/race versions and two different vehicles IF one cares to go make them.
It's more than a little ridiculous that changing the designation in a UI from "road car" to "race car" is all it takes to make these cars legit race cars for so many. Which brings me back to my original point, that people don't know or, like you, simply don't accept them.
And THAT is why the addition of a "Racing Mod" is a possibility. It's possible, because it's already there. All PD have to do is create a one step shortcut.