if it can upscale 1080 to 4k, does that mean I don’t need my 4k collection anymore? It sounds too good to be true.

Anecdotal so obvious grain of salt but:

It really depends on the kind of media. I have found that it does an AMAZING job with 80s anime where you basically had humans drawing vector art in a lot of ways. It doesn't work as well with the batshit insane "textured" 90s stuff and is hit or miss with the cg-enhanced >00s stuff. Same with live action. Something like The Fifth Element (but not actually The Fifth Element since I am pretty sure that was actively part of the training set) will look "a bit off" but a nice drama with "normal" costumes looks solid.

But also: "a bit off" mostly is for s tuff that I "know" what it should look like. And you might get lucky and see a weird artifact for a split second if you pause at just the right time. But it is really not that dissimilar from compression artifacts from a "poor" rip or the crushed bullshit that the streaming services call "4k".

So I still rip my UHDs. But if you are grabbing them off the internet and they've already been compressed to hell or were ripped from a streaming service that is similarly compressed to hell? I wouldn't bother and would probably stick with the 1080p versions.

Install Stremio with Premiumize. There's two main add-ons you need,most guides will point to them. From the top of my head it's Torrentio and Cyberflix.

Sound TV for twitch.

Smart Tube Next for awesome ad free, sponsor free YouTube.

TiviMate for IP TV (like once eoff $30 app)

I like Dreamy for a screensaver.

Mouse toggle if you need a mouse and want to use the remote.

Xmanager for file management. I can't remember why but it's the only one that can open zip or something like that.

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.

Install Kodi as well as the Jellyfin client.

Some older files just won't play otherwise.

Shield doesn't use AV1 so don't download any of those.

Just make sure it uses wired Ethernet. The WiFi module on it is doodoo.

Can't help you with any of the other stuff, but to answer your 4k question: No, you still need your 4k collection for best quality. Upscaled 1080p is still 1080p, it just makes it not look wonky on a 4k display. Any details lost due to the source resolution will still be lost, you can't create extra detail out of thin air.