The AI upscaling isn't magic, it's just a decent algorithm to get the best out of lower res content. tuned properly and combined with a TV properly configured to not oversharpen everything it looks excellent but is best left at the lowest setting.

My findings after using a 2019 shield pro since launch (also decrapified via ADB, and running projectivy) is that it is way too aggressive most of the time on the medium and high settings, leading to heavy artifacting and unnatural looking edges. for live action 720p and 1080p it looks fantastic on low and sometimes medium, but often over-sharpened on high depending on the content of the footage on high, for animation it looks down right deepfried meme sharpened sometimes unless you set it to low.

It cant make new detail where there was none before (it's not anywhere near the same tech used in high end GPUs like DLSS), its just a smarter, more content aware smoothing and sharpening filter, you can turn it on and off and bring up a side-by-side to see how it is processing the content to help you tune it and also to dial in your TVs settings to suit. if you use resolution switching in plex it will not be active by the way, that will force the shield to output 1080p and the TV handles the upscale at that point, so turn that off if you want to use the shields scaler.

on 1080p high bitrate content (bluray rips) it looks great, again usually on low or medium, high is too much most of the time, it gets very close to a native 4k rip of the same source if the source is a bit softer to start with, like many films with 2k intermediate masters where the advantages of 4k aren't fully utilised. Native 4k (some higher end films, many high budget tv shows, high value youtube channels etc) still look significantly better at true 4k than AI upscaled 1080.

So set the shield to Ai Low and set your TVs sharpness to zero or very low (mine is on 5 out of 100 for example in its movie preset, every one is different of course), there are calibration and test patterns built into projectivy (great feature!) you can use to tune motion smoothing, colours and contrast in SDR, HDR10 and DV as well as detail test patterns for upscaler and sharpness tuning so use them for the best experience.

One thing to note about the instructions on the website you linked for decrapifying via ADB, on the latest firmwares 9.1.1 and newer some of those commands will crash the shield, so go through them one by one rather than running the one line all in one, go through each line and if it forces a reboot, wait for it to come back and just go onto the next command. it wont do any damage but you have to skip a couple of lines to get it to work.