Terrible picture quality - Vizio d58u-d3 (2016 58" 4K) - any advice?

So I bought a Vizio d58u-d3 4k smart TV used when I took over the lease on an apartment, which is / was supposedly a very good bare-bones 4K TV for the price (review). The previous tenant bought it new and said it was always great.

Unfortunately, the image quality is terrible. The colors are very dim, sharpness and detail look like 720p at best, any fast motion often blurs or leaves traces. It looks far worse than the Hisense 1080p TV I had previously.

I've tried all five HDMI ports, I've tried different inputs (integrated streaming apps, Xbox 360 and my laptop), I've messed with the picture settings endlessly- all to no avail.

Are these problems indicative of a bad panel, or could something be busted inside the TV?

Thanks for any input!