I’m pretty sure this will be a lot of work. Even assuming you didn’t rename any files and all the torrents are in a single folder, you still need to re-search for every torrent file and add them to a torrent client that should automatically find the files in the target location and check them to see that it’s already downloaded. I’m not sure if all torrent clients do this so I would advise testing it first - when re-adding a torrent with existing files I usually add it paused and then force re-check to be safe.
In theory it should be possible to make a program that does this - automatically indexes and crawls torrent sites and DHT, fetches torrent meta info, then scans your HD for matching files to seed. This feature is a proposed one for the DHT crawler “bit magnet” so I’m hopeful that one day there will be a program that can do that.
I believe 4K is already basically there. I have a 50" 4K (2160p) that I sit 9 feet away from and based on the Nvidia PPD calculator, that makes for 168ppd, and according to that page 150ppd is around the upper limit of human vision. Apple’s “retina” displays target around 50-60ppd (varies based on assumed viewing distance), which is what most people seem to consider “average eye visual acuity”. Imo 4K / 150ppd is more than enough.