This a very helpful guide. Thank you for taking the time. Now take a donation for your efforts.
Typo in "Progress Bars in Loop" a code comment in restart_animation() says:
# scale = 1.1 when this step stars...
Should be # scale = 1.1 when this step starts...
In the simpler version the audio does not seem to pitch increment.
The font that you are using is causing <= to be displayed as ≤ which is less readable and I do not think Godot will accept it, though I could be wrong on the latter.