But to answer your question - I don't think YouTube will add ads to current-day YouTube Premium. The amount of money YouTube makes by me paying the $7/m is much more than the money they make by selling ads to me (we know this because many creators have themselves noted that the average YT Premium subscriber makes them more money (CPM)). It is also trivially easy for YouTube to increase fees by $1/month without (much) backlash, which would probably still make YouTube more money than showing me a few more ads.
I would not be surprised if they add an ad-lite "YouTube Silver", but I don't think that's the direction Google wants to go in; they'd rather you pay more for more things. You can't get YT Premium without YT Music right now (the price of Music is baked in), and Google pushes Google One on me a lot.