One of the reasons I used to keep (for example) every GT3 car I could find was to race big-grid GT3 races at tracks like Spa, with every car different. So, classic case of the grid fillers. Maybe CSP now spotlights the deficiencies in a lot of these badly-made cars, differences that weren't as noticeable before. But these days it's harder for older substandard models to pass visual inspection and they can stick out like sore thumbs out on the track, and that's ignoring other issues like BOP. Nowadays I find it easier and more consistent to mainly run spec races, or using cars with equal visual standards and properly BOP'd.
Like you say, testing new mods can be a tedious process, but it's nothing compared to deciding which ones to delete at a later date.
My advice to anyone building a collection of AC mods - cars and tracks - is to be brutally honest with yourself about whether you're likely to use it again, and if in any doubt whatsoever - delete it. You won't miss what you never had.
Also a lot should be based on what you like to use in sim racing, while back i binned all of my F1 mods, including RSS ones.
Its mamothly dull in real life and even more dull in a game (IMO).
Obviously if your a massive fan then (royal) your opinion differs and I'm cool with that as you should be with mine.
As I've converted more stuff I've deleted other stuff i don't use, rally tracks all went for example.
Just do what you want basically based on what you like, and if you have the storage just store it up.
Every so often i bin a bunch of stuff but mainly as its a type of car i don't like, then there is stuff like the DRM mod that stays there because the cars are cool, but they always without fail disappoint me when i race them.
I really should delete them.
Something else i'm keen to do these days is like with rally stuff, i load up DiRT, or open free roam Forza horizon etc.
This helps for me, to remove stuff that is just filler that other games do better.
This struck me when i was converting a plane into AC... and then i loaded up MSFS.
Just because you can, doesn't mean you should...
(BTW only quoted your post masscot to keep some continuity)