I'm not aware of 13" laptops that are UHD resolution. Part of it is that most 13" laptops are advertised as light notebooks and don't have dedicated graphics, and 13" doesn't need that kind of pixel pitch. For 15", I think most current offerings with a dedicated graphics card can handle UHD just fine (I know there are some arguments for the latest games and how much video RAM you need...in that case, you may need to reduce resolution for optimal 60fps newest games).
I recently got a Lenovo Yoga with UHD touchscreen, I7 processor, 16GB RAM, and 512GB solid state drive. I bought it to be my portable computer. I've never had a hiccup with 2D graphics. I also liked the 2-in-1 feature and that I can illustrate with a Wacom stylus. Windows 10 is also a better PC OS for 4K: it has some nice compatibility features for what apps you have that might not support "highDPI" (scaling fonts up to the bigger scale you set with Windows Desktop).
There are a few options out there for 13" UHD screens, but not many. The Surface products are all pretty close to 4K, though with a different (better, imo) aspect ratio.
I second your findings on Windows 10, also the usefulness of a touchscreen actually has me considering selling off my 13" to trade up to the 15" Surface Book 2 as I now never touch my desktop for editing purposes... the pen support on the Book is just way too useful, it makes editing much more enjoyable for me and it's barely slower than my desktop for most tasks anyway.
I’ve only owned Macs so I’m extremely biased. I would suggest a MBP with 16GB of RAM and a 500GB SSD for processing and editing images. The last one I bought was from Apples Refurbished Store and it arrived as new and hasn’t disappointed me in the two years I’ve owned it. I think that 8GB may disappoint you and cannot be upgraded by the user.
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