I have researched higher-resolution Ultrawide monitors recently, with particular attention to pixel resolution, and have not found any with Apple’s so-called “Retina” resolution.
I have been spoiled by high-resolution displays after using a 27" Retina iMac since 2014 and more recently a 13" M1 MacBook Pro. These feature pixel per inch (PPI) resolution of 218 ppi for the 27" iMac and 227 ppi for the MacBook Pro. The text resolution of both is extremely sharp, even when scaled or magnified to a larger font size.
For me, using a lower-resolution display for productivity work - text, spreadsheets - is tiring and unpleasant. This is quite personal - some users are not bothered by lower-resolution displays and value other qualities such as high refresh rates (especially prized by gamers).
Your suggested Dell 49-inch Super Ultrawide monitor U4919DW has a resolution of 5120x1440 and a ppi resolution of approximately 109 ppi. The height of the monitor’s actual viewing area is 13.2", relatively short for productivity work such as text documents and spreadsheets. Since you mentioned preference for a Retina display, I would point out that the resolution of that monitor is nowhere near “retina”.
You might have a look at Dell’s 40" Ultrawide U4021QW. The PPI resolution of 140, while not “retina” quality, is relatively good and much better than the Dell 49" Ultrawide. Another good feature of the U4021QW a viewing area height of 15.5", similar to that of standard 16:9 ratio 27" monitors and a good height for viewing text documents, spreadsheets, etc.
Another consideration would be one of LG’s 34" Ultrawide monitors, the 34WK95U. Like the 40" Dell mentioned above, the resolution is so-called “5k2k”, or 5120 x 2160. With a vertical viewing area height of approximately 13.5", that translates to a PPI resolution of approximately 160. The height of the monitor’s viewing area is only 13.5", though.
Like you, I would like an Ultrawide monitor but have found none that approach Apple’s “retina” resolution. My order is in for a pair of Apple’s Studio Displays, to be mounted side-by-side.