Personally I use UnHook to completely hide all recommendations, merch/music/offers/etc. ads, replace the default homepage with the subscriptions page and generally clean the UI:

https://addons.mozilla.org/en-US/firefox/addon/youtube-recom...

In addition, I use the Stylus extension with a custom stylesheet to restore the UI to the roughly pre-2016 look. This has allowed me to essentially treat my Youtube experience as a RSS feed viewer, which I feel is a much healthier experience.

However, by doing this you do lose out on the discovery factor that Youtube is known for.

Sadly not from the apps without sideloading modified versions. Not only is the quality absolutely garbage but I also noticed that my brain is susceptible to them and I tend to doomscroll if I ever land there. I really try so hard to excise all the short form content stuff from my life but even with extensions and everything, the web version is not great enough to replace the app completely.