OK, here is an update from me (as promised to @JoshFossgreen):
(Before I forget it: nice playing on “43” during the outro!!)
So, your diagnostic approach in the video was spot on. First, I had to adjust my thumb with respect to the strings, as I was hitting them way too parallel (i.e., no angle).
Second, I clearly suffered from a thumb that wasn’t relaxed at all, and basically killed off all tone right after hitting the string. This has improved, but still some way to go.
However, there was a third aspect that is almost more a “psychological” thing… Because my sound was very percussive and very “clanky”, I didn’t like it at all, which created a negative feedback. In order to avoid it I played very softly, which just aggravated things really… I also played softly because with my wife in earshot, it felt embarrassing, like I was torturing a string or the entire bass or something. (Obviously, playing with earphones doesn’t help, as all the percussive “noise” is still audible for anyone close by).
Recently, I had the house for myself a couple of times and finally cranked up the amp and slapped much “harder” (with intent), which allowed me to better understand what I was doing right and what I was doing wrong. Having figured this out, I can now also get the right combo of percussion and tone less violently
Still a long way to go, obviously, but your video was immensely helpful to get me out of the slap funk I was in (pardon the pun)! Thanks!!