The kinds of videos I always make sure to watch in a private tab are things like:

• viral videos
• videos recommended by a friend
• "how do I fix this broken toilet" videos, or something else that I need once and won't ever look at again

With that approach, my recommended videos feed is still trash, but at least it's vaguely on-topic trash. I don't get recommended 500 plumbing videos because one time I needed to know how to fix my toilet. It doesn't take the fact that I watched a video that 500 million people also watched as meaning "I want to watch all the popular videos" or "I have the same tastes as everybody else who watched this video".

I can always tell when I screwed up and watched an off-topic video in my normal account because the recommendations go to shit for a while.