Yes appears display link only really works at 1920x1080 and anything else gets blurry. Really n point trying to use Displaylink on a 4k monitor as you have found. 1080p the text is really to large but any other resolution other than native is lowres and native has too small text because of the way the Mac scales. Looking forward i hope the m1x has support for dual displays as this will solve the problem finally. (I hope).