On your tire pressure question, there are several online calculators you can use. I like the Silca one here - if you give your name and email address so they can spam you a bit, you can use the full "pro" version.

Is it only really bumpy 5 hour rides where you have the ass pain? Bike/cleat fit, saddle type, pedaling motion, etc are big issues. I found that a bike fitting that ended up moving my seat up and my cleats back really reduce knee and butt pain on long rides, and doing hip flexor stretches really extended my pain-free time in saddle after that.

I have never ridden the Giant, but I have a carbon Trek Domane that has "compliance" bushings in the seat post and head tube areas. It definitely smooths out road buzz but on the few times I've ridden it on gravel (with 32mm tires at 70 PSI) it certainly wasn't as forgiving as my steel Jamis with 40mm tires at 45 PSI.

I may just not very sensitive though - I did 123 miles on a rented aluminum bike on the first day of the 2019 Seattle to Portland road ride and really couldn't tell that much difference between the Domane and the Fuji ride (nearly identical geometries). I also weigh 220, maybe much of it is padding I carry around on my rear...