On the 35" the text is too small to read comfortably for any length of time
Text size has no relation to the display size. Text size is generally specified in "points", where one point is approximately 1/72 inch. If you find the text too small to read, the obvious solution is to increase the size. Display size affects how much text you can display given a certain text size. E.g., you might get 40 lines of 10 point text on a 24" monitor, and 45 lines of 10 point text on a 32" monitor.
I don't see how reading on a 27" is going to work unless you increase your font size which reduces the benefits of the higher resolution.
Why wouldn't reading on a 27" work? A long time ago, I had a 15" CRT and was able to read text on it without any problems. And even further back, there were 9" screens, and even smaller ones. You just couldn't get as much text on them (e.g., 40 columns across).
The benefit of higher resolution is that text is sharper, since you can use more pixels to draw the characters while keeping the same point size. E.g., instead of using 8x12 pixels to draw a character, you can use 16x24, which looks a lot better. It's even more noticeable if you work with Chinese/Japanese/Korean text, where the characters are much more detailed than the Roman alphabet. Some characters (such as this one) turn into an indistinct mess if you have to squeeze it into a 12x12 pixel cell, but if you have 24x24 to work with, it looks a lot better.
In any case, this Dell monitor sounds interesting... I was considering their previous 4K 24" monitor, but the way it faked being two half-screens (to work around HDMI limitations?) seemed annoying and glitch-prone, and I heard that the next generation of monitors wouldn't have to do that. I currently have a 24" monitor, and am looking for something the same size, but I suppose 27" isn't too much bigger.