I've currently got OEM rails+cross bars, but MAY be changing out the crossbars to niserack or LFD... probably keeping the stock rails.

Looking for a cargo box for road trips to store some extra bulky, less-used, and dirty gear... and in winter for skis up to about 190cm in length.

I've come across a few discussions about max cargo boxes, hitting the rear hitch (which doesn't have adjustable height/stop-position...) so I'm a bit overwhelmed combined with tons of random used options. I've also heard the OEM crossbars can flex, and it can be difficult to secure boxes to them without risk of hitting roof (it seems weird what on earth is the point of OEM bars then?)

So, yeah... any advice for a cargo box with acceptable storage to throw random items and skis in winter? Or how to actually go about selecting a box myself and knowing what to look for/avoid?

Can't really advise on the box because I don't have one but I have a basket up there sitting on 4 OEM cross bars and there isn't any flex that I notice when it's loaded with my tent, BBQ and chairs (easily 100 pounds). I guess you might need the LFD option if you're putting a lot of weight up there though, plus they have awesome options on the product for securing things to of course compared to the stock bars obviously... maybe check on the box option with Rack Attack in your area... that's where I shopped they seemed to have every available option in their software.

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I have a NiseRack on my t4r with 4 crossbars. I have had 500 lbs up there (rooftop tent and 3 adults) and there was ZERO flex on the rack. Super solid. I would recommend a nice Thule Pulse Alpine. Should meet all your needs.
Thule Pulse | Thule | USA

This subject comes up a lot, so rather than post some pictures you are welcome to go into my album page and look at how the Yakima 16 box fits
With the Yakima towers and cross bars