Pixelated/poor resolution with use of Dell 49" ultrawide display

I purchased a Dell 49" ultra wide display (U4919DW) in hopes to streamline and replace two monitors I currently use with my Mac Pro (Late 2013). However - when I plugged it in and turned it on - the resolution is HORRIBLE. Text is barely legible - you can see the square colors/shades along edges of windows and fonts.

I've tried:

• Different connections (HDMI, DP to MiniDP)
• Different wires for each connection type (thinking it was the cables)
• Confirmed there's no zoom in from the Accessibility function
• Changed resolutions up and down

What DID work was to plug in the monitor with two different cables/inputs and used the monitor's PBP function - simulating two monitors connected side by side. [I've also used my 2018 MacBook Pro's USB and that works fine as one wide monitor]

The downside is that if I wanted, for example, to use the monitor and split it into thirds - I can't have a window/program in the middle third - it has to be on one half of the screen or the other - the image won't straddle the middle.

Is there a way to make the monitor work as "one monitor"? I have the stock AMD FirePro D500 in the Mac Pro and operating Catalina 10.15.5

Thanks