I recently just went through this exact process. We settled on the GMC Canyon, which is nearly identical to the Colorado in every way but with a bit more of a premium interior. Be warned though, stuff that bolts to the front of the Colorado might not work for the Canyon as that IS a difference; the front fenders and grill are a bit different. Everything else is the same, so it might be worth a look depending on your plans. The downside to the Canyon is no Bison package, which for us was OK as we plan on aftermarket suspension that will better suit our needs than what came with the Bison; the Bison has a lower payload than a stock Colorado/Canyon, and we wanted to maximize the legal load limits.
Plus, to be perfectly honest, we got a screaming good deal on the GM as it was a 2017 with less than 6000 kms on the clock (Basically new; just had it's first tire rotation the other day). We walked away for mid-30's, whereas the Gladiator was at the time over $75k locally. I can buy two Canyons -- which I am fairly sure will get me to 500k! -- for the price of one Gladiator. That will come down in time....but how long?
I wouldn't worry about the frame bending issue, because from what I can gather, it's not an issue. The vehicle is designed to be safe on the road, and 99% of time your risk from being rear ended will be far greater than the risk of bending the frame off-road by towing a trailer (which is a whole other topic; lots on Expo seem to love off road trailers but I am not a fan for my application due to the increase risk to broken stuff when towing).
Reliability between the ones you mentioned is very competitive in aggregate. There are always individual cars from every mark that are lemons, and sometimes we shout louder about some lemons than others, but almost all of the data you can find on discussion forums is anecdotal, and it's hard to get a real sense of reliability from stories. Reliability is subjective -- for some folks, they only ever want to turn the key and change the oil, and anything other than that means a rig isn't reliable. For others, they are comfortable with changing all the wear parts regularly, and if they do CV joints every 30,000 kms, that's just "cost of maintenance" and not a reliability issue. Therefore the stories we see are not a good indicator of overall reliability -- there are too many perspectives and too many variables in the stories to really compare apples to apples.
The people who pay attention to this stuff and aggregate the actual reliability rank the Colorado very closely with the Taco (which is the market leader in many ways). For instance, RepairPal ranks the Taco at 4.5/5, and the Colorado at 4/5, with the half-point being made almost entirely on the 'annual cost of repair' being lower on the Taco by 200 dollars per year. 200 bucks is not significant enough to make the difference in my book. In all other ways, the two trucks were very close (within a percentage point), with no clear favour to one or the other).
As far as the interior fit and finish, like I said the Canyon is very nice -- far nicer than our JK. My chevy Silverado is not nearly as good, though, from a 'creature comfort' perspective. But there is a difference between 'spartan' and 'flawed'. My chevy is Spartan, but clean and nice -- a spartan interior can be an asset in an overlanding rig, as it's easy to clean, not many crevasses to trap dust, etc. However what you described on the Bison you test drove is flawed, and if I were you, I'd demand to see a new bison OR see the cosmetic issues fixed/reflected in the price. Not all Bisons are made Friday Afternoon with weekends on the mind of the good guys and gals bolting them together, after all!
None of the trucks you have mentioned are likely to make it to 500k miles on their own, depending on the use of them. All of them will need varying amounts of TLC along the way. But all of them WILL make it to 500K if you are willing to put that TLC in, up to an including changing major parts. From that perspective, I think you are better off with the GM out of the list you have provided. Given there are 4 trucks that are essentially the same (Holden, GM, Chevy, Isuzu) and all of them are selling very well, both at home and abroad, those critical "keep it on the road" parts should be fairly widely available for some time.
