Need help with my V-brakes

Hello, I live in Berlin and just bought a secondhand folding bike for my commute. The seller told me I'd need to adjust the rear brake, but even with the brake pads fully tightened (see photos), I need to tighten the lever so much that it almost touches the bike's grip. Everything seems to be in order, except for the handlebar thread that allows you to increase the pressure, which won't stay in place (it seems the internal thread is stripped). I don't know if I should buy a new lever or if the fault lies in the cable. How do I have to press the brake so hard for such a narrow cable distance? I'm sure it's something silly and I can't figure it out (I'm no expert on this). The front brake works without any tension issues, and I can tighten the aforementioned thread. I hope you can help me find the solution to my problem with the attached photos and video. Thank you very much, and best regards.