OxygenOS sounds like non-stock Android to me tbf, the G5 Plus runs a close to stock Android too (but they've not felt the need to rename the OS!).
The camera on the G5 Plus is the same sensor as the S7 but it lacks the OIS iirc
A sensor doesn't always translate directly to picture quality. The one on the LG G5 and G6 werent what we'd consider flagship grade, yet they take some of the best pictures, beating out the S7 in many areas.
Oxygen doesn't add any bloat like the other manufacturers, namely Samsung. At one point they had an app for everything Google already do. On my Nexus 6P, i had 80 apps installed total. On my S7 edge, there's north of 110 (same apps downloaded, the extra is from Samsung).
OPO 3T lacks 2K screen, front stereo speakers, microsd card slot and hi-res audio.
If you're referring to 1440p, having owned many 1080p and 1440p devices, i still feel 1440p is unnecessary and not worth the tradeoff battery wise (though in reality, at least with Samsung, the displays are more efficient than most LCD 1080p panels, so in reality I'm not fussed).
The number of micro SD cards I've had fail on me, or refuse to transfer to the computer, or don't work well for having apps installed on them, I'd rather just pay more for a larger storage.
Front facing speakers, while great, aren't what is consider key for what I use my phone for. I don't listen to videos through the speakers enough to sacrifice something else. Nice, but i didn't miss them that much going from the 6P to the S7 edge.