Youtube-dl has the option to download only videos which are uploaded after a specific date. So you could run the script once a day and download the videos from this day.

Youtube has RSS feeds for channel. You could use that as a trigger to download the video as soon as it gets released. I hope this can be helpful :)

Switch to https://github.com/yt-dlp/yt-dlp its better, its an updated version that is mantained. Right now I dont remember by memory the arguments. But if you read the documentation you can setup a file that contains a list of already downloaded videos. I used to use it to download playlists that I made and it only downloaded new videos. Read this thread, its should apply to youtube-dlp with a whole channel: https://askubuntu.com/questions/673442/downloading-youtube-playlist-with-youtube-dl-skipping-existing-files

https://github.com/rebane2001/sensible-yt-dlp-archive-scripts

This would help you get started. Run this every x hours.

I believe there will always be a way to watch these videos ad-free. I can't imagine they will restructure the video serving infrastructure to force ads in a way that can't be blocked as there is so many devices and official YouTube clients (gaming consoles, Smart TVs, etc) that would have to be updated. However, I do think they might make it more difficult.

youtube-dl is the way to go. One thing you could do is keep a text file containing video IDs that you have already downloaded and before downloading a new video and append the new video ID to the list. Another way is you could also set the file name to the video ID to and check if it exists before downloading as well.