I just want to point out that unless you are using the most advanced settings, a lot of the features developed for the 4K have been backported to the RetroTink-5X via firmware updates in the last two years, and apparently it only stopped because the firmware is as big as the FPGA in the device can handle. The CRT stuff looks great even at 1440p.
As cool as the 4K model is, I don’t even fully use the features available to me because spending 12 minutes fiddling with the remote to get the perfect settings before every game would be a deterrent to playing anything, so I don’t feel the need to upgrade from the 5X (thankfully, because the combination of price and exchange rate would kill me)