One thing I would add is that this video made it seem as though English is relatively unique in being stress-timed.

Indeed, it isn’t - it’s a really common feature across the world.

which, combined with English, suggests perhaps this is an ancestral feature of the Germanic family?

IMO it’s possible, but unlikely. Two reasons:

1. PGerm had a 3-way vowel length contrast, so morae were likely a big deal in the language. Odds are that the language was either mora-timed (like Latin) or a middle ground between mora-timed and stress-timed.
2. Isochrony changes really fast, diachronically speaking. Portuguese is a good example of that - all dialectal variation is ~600yo, and yet the language shows dialects all across the syllable-timed vs. stress-timed continuum.