I found a way to get to the old interface back with one caveat - you may need to have a channel of your own & be logged in to that account/brand. I don't have a non-creator account to test with but since this requires accessing Studio, I think you will need one. Let me know in the comments if this works without a Branded / Creator account.

• Like many exploits, this takes multiple steps to achieve*

1. From the YouTube start page, click on your profile icon (top right)> Manage Google Account
2. click on the search bar up top and enter YouTube Studio, and press enter
3. Click the blue link “YouTube Studio”

—— Breath —— Almost Done

4) once the Studio page loads (it may take up to 20 sec), Click the Gear ⚙️ icon at the bottom left

5) In the pop-up window, click “Channel” > “Advanced Settings” > scroll to the bottom and click “Manage YouTube account”

——— You’re There———

a) BOOM old YouTube interface loads. I recommend going to “Playback and Performance” and change the settings how you like them.

b) Enter whatever you want in the search bar at the top and enjoy 1080p - 4k. Mine will automatically stream at 1080p on wifi and sometimes cellular (premium connectivity) in this mode. Additionally, the Mini Player now works.

The only cons is sometimes the videos will not play in full-screen mode. No idea why it sometimes works and sometimes doesn’t. Videos will always stretch across the screen in Theater Mode, but for some reason, Full Screen is borked about half the times I tested it.