I have a 3.1 modem but my ISP only has 3.0 speeds as far as I can tell. 1000/100 is their highest plan so the extra doesn't really do anything.

My modem is 32x8 and I can see in the UI that only 4 of the 8 upload channels are actually bonded to reach that 100, which is half of the 200 that 3.0 can theoretically do.

I already do this. There are several apps for it but I really like Pinchflat because it has metadata settings for Jellyfin.

All I have to do is add a video to a playlist and it gets automatically downloaded. I use it for archival but there's nothing stopping you from making it the only way you consume YouTube content.