I used to think jailbreaking was the only way to do this, and mourned my adblocker tweaks when Apple made jailbreaking practically impossible. But turns out sideloading on iOS is a pretty easy alternative: just install AltStore via your computer, sign in with your Apple ID, and then import and install the YTLitePlus .ipa from their GitHub[1]. This gets you a YouTube clone with adblocking, SponsorBlock, custom UI controls, and all sorts of other quality-of-life features. You can even sign into and sync with your existing YouTube account.

The only downside is that free Apple accounts must renew their certificate in AltStore (while connected to their computer's home network) once a week, or else it'll all be deactivated and you'll have to reinstall AltStore and YTLitePlus from scratch. But you can pay $99 for a year-long developer account, set a recurring reminder to renew, or worst case YTLitePlus makes it easy to export your settings so you can quickly restore it after reinstalling.

[1] https://github.com/YTLitePlus/YTLitePlus

youtube premium for a year is probably cheaper than an apple developer account though