So, the main issue I am running into is the lack of space in the truck for mounting regular depth subs.
Welcome to trying to put an audio system into a pickup truck.
"Space makes bass" and "there's no replacement for displacement" are two common themes we have in audio. This goes along with "Hoffman's Iron Law" which states you can have high efficiency, good low extension, or small space, but only two of these.
That said, almost every sub out there will be louder/deeper/more efficient if you can build bigger than "recommended" box size, but recommended should perform OK. Cone area is good, but what we are really after is displacement. A smaller cone with a longer stroke can move just as much air as a larger diameter cone with less excursion. Most importantly is that every woofer is different and they must be properly paired to an airspace that compliments their mechanical and electromagnetic specs. You can get the "best" woofer on the market and put it in the "wrong" box and it will perform poorly.'
The "shallow mount" woofers are definitely going to be excursion limited but may well get you buy. IF I were buying one today I'd be looking into the Dayton over to
PartsExpress. Dayton stuff I've used in the past is good value and quality and they have honest published specs and reviews.
You must figure out PRECISELY how much net volume you will have to work with, paying careful attention to mounting depth and physical dimensions of the woofers you will consider then shop around that. Once you are there, decide what coil options you have (what your final impedance load will be) then plan an amp to properly feed them.
I've seen several good looking pickup truck boxes that go under the rear seats using multiple 6" or 8" woofers. They are sold for a steep price for the ones that look super nice but if you're handy with woodworking you could certainly DIY something to get the same job done.
I’ve been very focused on subs in this thread and just started thinking today that if I use the existing head unit in the existing factory speakers even if volumes are not an issue I’m wondering if I’m gonna be disappointed with the sound quality of the highs?
Very possibly. The idea of those cheap-o tweets seems like a reasonable gamble. If you """need""" more you can always replace/upgrade factory speakers and/or amp them down the road.
The s series 10 inch recommended box size says 0.58 cubic ft. Interestingly, the R series 10 inch also says box size of 0.58 cubic ft. Is this correct? Seems like R would need bigger? There is likely much I am misunderstanding
Type R lineup is all very nice. I've only heard type S components and wasn't terribly impressed, I'd have low expectations for the subs in that line and would avoid them provided you have the ability to mount the bigger subs.