Sorry I didn't see this question earlier, I am just starting to get back active on the board after a looooooong absence.
With my wheel/tire set up sticking out quite a bit I also wanted to get some extra coverage with fender flares. But I ran into the same problem where nobody made flares with substantial coverage so I ended up going with what was the only available product which was ATS C795-611. These provided minimal additional coverage, basically the same as with some special Trailblazer models that came with flares from the factory. They may have even been the supplier to GM at the time, I don't know. Here is what my setup looked like at the time:
Unfortunately I cannot find any reference to ATS on the Internet. Did they go out of business? But if you can do some extra digging and find out what the status is of this product, I have a second part to the solution.
Many years later I happened to come across a product that peaked my interest as a possible addition to the ATS flares - Flexy Flares from Pacer Performance. Particularly their product number 52-118 which gives 2.5 inches of extra coverage.
Here's the product page on SummitRacingThe problem was always going to be the flare on the rear door. In fact, on the ATS flares, mostly everywhere they are somehow attached to the sheet metal from the wheel well side (forget exactly how, I think there are stock holes that they take advantage of). And then they use double sided tape against the body to make them even more sturdy. But on the rear door, they only use double sided tape because there is no surface area on the wheel well facing lip on the door. Take a look, you will see what I mean right away, there's only a thin edge there.
So with the Flexy Flares, how could that work? Well, I found that I could attach them into the ATS flares themselves! The ATS flares are solid enough to accept the screws provided (except I swapped them out for stainless steel) as long as you don't drill them too close the outer side of the ATS flares - it can bulge the flares out which doesn't look too good.
I am going to my storage where the Trailblazer is parked later this week and I will take some pictures of what the whole thing looks like with the Flexy Flares addition.
Hope this can help you out somehow.