Recently my youtube client stopped working with an um, dummy account, used for partial cookie access. I had to do the ol log in and cookie refresh, to find out that all viewing has been blocked due to...
One angle that still works in some cases: browser userscripts. Install a manager like Tampermonkey or Violentmonkey and one of the YouTube age gate bypass scripts, which people report working in browser without logging into any account. Worth a shot before giving up.