The easiest solution IMO is to download Firefox for your phone and use that instead of the YouTube app. I don't watch a lot on my phone, but that's what I did. Just install the Ublock extension like you would on a desktop and you're all set. Then you can also use that instead of Chrome and free of ads on the web at large as well.
The one unfortunate thing is that you can't uninstall the YouTube app, only disable it, and every time you go to the site it will ask you if you want to open the app instead.



These kinds of things are often A B testing to see what they can get away with - especially in countries like the US where consumer protection laws basically don't matter. It's kinda like when they raise subscription prices in one country but not another, but with showing some people in that country a certain amount of ads per watch time vs another group with a different amount of ads. They see how much they can get away with before people start complaining, and then what they can get away with before people start cancelling.
I watched a video just this morning talking about how YouTube has built-in systems for similar A B testing with video thumbnails to tell creators which gets more watch time. You give it 2 thumbnails and it randomizes which one people see, and then tracks click-through rates and watch time percentages before giving you a result of which one performs better.