Maxi-Rant, rest in the first comment!

Yay, I've caught up with my "watch later" list on YouTube! Next thing: Just quickly go through my subscribed channels and add old videos that I haven't seen yet to the watch later list so that I have more stuff to watch the next months. The easiest way to do that is to go to the "all uploads" playlist of the channel (that is luckily always linked now, it used to be hidden sometimes) and use "add all to" to get them on my playlist. Then sort out the stuff that I've already seen and turn on automatic sorting by date, easy. Yeah...
Firstly, in the new design there's no "add all to", I have to go to the old design. For my own playlists, there's a handy "edit" button to do that, but on other pages I have to do it manually. Luckily I have set Ctrl+Shift+1 as a shortcut for "&disable_polymer=true" long ago.
Next surprise: On "all uploads" playlists, there is no "add all to" button. It's on every single other playlist on YouTube, including "liked", "watch later", "favourites" and so on, just not there.
Fine, I'll just abuse my subscription playlist script that I already have by making a copy of it, putting the channel IDs in it and setting the last execution date to 1.1.2001. Little problem with that: Google apps scripts can run for at most 5 minutes and the YouTube API restricts it to add one video per second. So it doesn't work for more than 300 videos. I could now try to split it up by dates, but I didn't write the script myself and I don't know how it sorts the videos to add, so I'll just google for another solution instead.
Found one: Go to the video overview of the channel in the old layout, Ctrl+Shift+I, paste this little Javascript thing and it automatically clicks all the little clocks that add the video to the watch later list. Yay, that works! Ok, i'm restricted to 5000 videos, because that's the maximum size of a YouTube playlist, so I can't immediately add all 8000+, but whatever, that's a minor problem and I'll sort out later anyway. Still another little problem: For some reason I can't automatically sort the watch later list. Because that would be too easy.
But whatever, I'll just use "add all to" from there to add it to my creatively named "WL" list. If that thing is restricted by the same rate limit of 1 video per second, it should be done in about 1½ hours. A bit long, but hey, I'm dealing with 5000 videos. Waiting 2 hours... Waiting 3 hours... Nothing happens. It would be nice if it at least added them one by one, but no, it waits an eternity and then adds all at once. At least in theory, right now it does absolutely nothing.
Shortly considered running it for more hours or even days on my Raspberry Pi, but that thing already struggles when using Chromium normally, I shouldn't bother it with anything that has to do with 5000 videos.
Ok, what else can I do then? Googling, trying out different things, mainly external services that have their own concept of "playlists" and can then add them to an arbitrary playlist later...
Even tried writing my own Java program with the YouTube API, but after about an hour not even the example program in the YouTube API tutorial worked (50 errors and even more open questions, woohoo), so I discarded that idea.
Then I discovered "DiskYT". Everything looked like it would work and I'm still convinced that I can do it with that little pile of shit. Why is it a pile of shit? Well, for example the site reloads itself after a while, so it can at most add 700 videos to a playlist. Also I can't just paste the channel link (even though it recognises those links, but just to show an error message that it can't copy from channels). I can't enter/paste URLs, I have to drag them. The site saves absolutely nothing (should in theory work, but in practise it doesn't), so I have to re-drag everything on every try. In one network, the "authorise YouTube" button (that I have to press again on every computer) does absolutely nothing ("inspect" reveals that there isn't even any action bound to the button), in another network the page mostly doesn't work at all or the button to copy from playlists is suddenly gone or other weird stuff. Luckily I have the WiFi at home, there it works in theory. But just on my desktop PC, no other device, wow. I tried to run it on my new laptop, but it's so new that it still has the preinstalled OS and there I can't deactivate going to standby when closing the laptop, so while I expected it to add 5000 videos, it instead added 4 and went to standby. But doesn't matter, because it would have failed at about 700 anyway. Every time I try to use this website, I get new problems, but it seems to still be the best option, because everything else just doesn't do anything. This page at least got to 700 before.

Continuing in first comment!