Did you imagine buying a 4K TV for less than $400 a few years ago? This TCL unit has very positive user reviews, it's 55" HDR10-capable and its smart TV portion is powered by Roku. Demand is high, but you can still buy if you're willing to wait for it to ship.
I had toyed with the idea of using a TV as a monitor on a couple of different occasions in the past decade with no success, but with the advent of affordable 4K sets using a TV as a desktop monitor is finally feasible. Here's my experience during the past few months.