Life can be so simple

Stop playback at End time

Select audio option:

With Video+Audio

Only video

Only audio

Paste Youtube Url
Let it play, navigate with mouse, whatever
Play/Pause with SPACE or k
Navigate frame by frame with . (DOT) and , (COMMA)
Navigate second by second with the Arrow Keys (Note: If the player is focused, it will skip by 5 or 10 seconds and won't catch Start/End key bindings)
Set Start and End Time with s and e
Jump to start with (capital) S
Pressing the DOWNLOAD button will generate a yt-dlp command
(It needs to be installed and added to PATH so it can be called anywhere from the command line) "What?"
Copy it and execute it in on the command line, it will download the video/audio in the current folder by default
If the time is displayed as 12:00 AM/PM, instead of 00:00, this is due to your locale settings, I can't change that with html/css. I find it very annoying.