Locking for rule 5.

It's not a super elegant solution, but whenever I come across age-restricted videos ytdownloader.io has managed to download them.

No, only because on google's end, if the account is not verified, they will not serve the video so nothing you do on your end will capture the video, since there's no video to be captured. If you're unwilling to create a throwaway account to view those, just give them a miss and watch anything else your heart desires that doesn't currently require an account to view.

the content I look for doesn’t exist on other platforms.

Can't dangle that carrot before us without saying what it is, I'm afraid

If the video was uploaded before youtube forced accounts for age restricted videos (I don't know when that was exactly), you can check on web.archive.org if the video was archived and use the archived url in yt-dlp. If the video was uploaded more recently or there is no archive old enough, I don't know of anyway to download it unfortunately

FYI you should use Grayjay if you don't already, it's excellent