Thanks for the confirmation on the behavior of the voltage gauge. I assumed it to be correct until I noticed that the turn signals drag the gauge down significantly yet the hazards do not, indicating that it's a poor representation of actual battery voltage- but that was just an assumption on my behalf.
As far as the 7506 bulbs:
My goal when trying to find the best LED tail light bulb was to use one that's easily available at local stores.
I somewhat succeeded.
The 7506R, (the suffix indicates a color, R for red, A for Amber) can be found at local parts stores, but they seem to be in low demand and supply. They will likely have to order them, but they're most definitely available.
I have not tried the amber yet, and based on the reflector design of the rear turn signals, I do not think the two diode type will work very well. Thankfully they make ones that have many LEDs in a cylindrical shape that will act more "incandescent" and produce a good beam.
Edit 12/13/21: I have now added all of the above info regarding taillights, and more, to the Wiki! Feel free to edit and add whatever you'd like if you learn anything else about the subject, especially alternate part numbers as bulb part numbers are an absolute tangle of compatible and equivalent parts.