Differing voltages on pin D7 and D8D6D5

Yes absolutely. I now know that different LEDs have different current requirements.
What I did not appreciate is how this will affect the voltage at the output of the digital pin of the Arduino. I know this is a thing, but I have not quite got my head around why the voltage of the output wold be affected (measuring common ground to the output pin).

Relating this back to general electrics, operating a 3kW kettle or a 60W lightbulb will have no effect on the 240V (UK) measured voltage at the socket outlet. I've not quite got my head around why this does not translate down to the Arduino's 3.3V and a couple of LEDs....

Look at it this way. What happens if you hook up a big light bulb to a battery that's almost empty? That's right - the clamp voltage of the battery drops. It's because the battery cannot supply the power that's attempted to be drawn from it. I think that's what you're seeing, because you're attempting to draw more power from the microcontroller's output pins than you're supposed to.

That's why your socket outlet example isn't accurate in this case. With your typical wall socket, you assume it has virtually infinite supply capability (within the limits of the fuse on the outlet). Like I said before, a microcontroller is * not* a power supply...

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Yes that makes sense. Thanks for your explanations.
I have learnt from this, and although I made this mistake, it's not something I'll repeat.

Trying to source some 3.3V MOSFETS now to continue my learning...

Always nice to have. In this application, something very simple such as a 2n7000 (2n7002) or bss123 will work OK; it doesn't have to be a dedicated logic level mosfet or a high power one.