It works for me in practice. YouTube definitely are capable of linking separate accounts in the back end, but in my experience they don't do so for recommendations. I have a separate account specifically for watching a given type of content and I don't receive any of its recommendations on my main account. Now, whether or not they use linking to give advertisements or sell data I'm not sure, but at that point they can link based on IP anyway and we're talking about a separate issue.

Yeah this is a very common strategy of language learners. Have a seperate YouTube account where you only watch content in your target language to keep the recommendations clean

Maybe get a cheap MVNO SIM like Ting Mobile and use that to register?

I know I've had google accounts that didn't require any re authentication in the way of phone numbers for going on years so you can likely discard the number after you show you are human.

Won't it still also require a credit card for YouTube to believe your age?

I'm sorry, I haven't registered a new account in a while. I tested an account I've had since 2016 that I made with a ex employer's landline phone and I've never registered a credit card to. It is able to play age restricted video by confirming that I want to and nothing else. It's under a fictional name and birthday etc.