P.
My best algorithm for choosing monitor is this: if you need to use windows scaling with given PPI then get monitor with lower resolution until 100% is comfortable then do not go any lower.
For most people sight it will be 1440p for 27", for many even larger 1440p monitor.
If 2160p can be used comfortably at given size then 1440p will have visible screen door...
I disagree. Higher resolution with DPI scaling helps give you better text rendering at the expense of desktop space. I even use 125% scaling on 1440p 27" because I find it more comfortable to read, not because I have bad eyesight (I've had laser surgery to fix it). With pretty much any 4K screen you are looking into using scaling on the desktop. Bigger 4K screens are the best of both as you can have a ton of desktop space while still having comfortably sized text without using zooming features in the few apps that support such a thing.
My beef with Windows DPI scaling is that it does not support scaling levels under 125% without using the custom resolution which only works for all displays. 110% scaling would be perfect for me on 1440p 27" but then my 4K 65" TV has unreadably tiny text.