My Shield is a 2015 model, so it does not have any upscaling settings.
I have the resolution set to the default: 4K 59.940 Hz, YUV420 10-bit Rec 2020 HDR10 Ready.
In Advanced Display Settings, Match content color space and Display HDR content when available are both enabled.
DVD rips, MPEG2 480p, look OK. Just as good as when watching with Plex Desktop or playing the file with VLC on my desktop PC.
It is obvious the video is from a DVD. No amount of upscaling is going to make it look like HD video. But it still looks good considering the source.
You might check the TV settings. My LG came with all their options such as “TruMotion,” “Real Cinema,” etc. enabled. It gave the picture the dreaded “soap opera effect.” Once I turned off all such settings, the picture was watchable.
Those settings would affect all video, but there might be one that affects SD video more than HD/UHD video.