Trying to design a switchable LED circuit with no experience or skill - please send help - PART 2: I set a transistor on fire, please send more help
I have solved the problem!! it totally works and I am STOKED
The guy who answered this previously and deleted his comment was totally right - all that was missing was a ground connection from the emitter pin to the RPi (See Scheme):

I also swapped to an NPN transistor (S8050) and followed a wiring diagram from a tutorial I have for an active buzzer so I was sure all the current and components were the correct way round.
I've had everything plugged into the mains now for like 10 minutes and no transistor fires so I think I'm all set :) here's a photo of everything together:

Next step is to put more LEDs in parallel with more transistors so I can selectively activate the different wavelengths I want for my experiments.
Special shout out to this guy:
https://www.youtube.com/watch?v=IG5vw6P9iY4
This video pretty much pointed out the only difference between my circuit and his (besides the power supply) was my lack of a ground connection, and low-and-behold apparently that was all I needed.
Sorry to anyone that I annoyed with my question, I'm very new to this and just trying to learn. Thanks for all the pointers and tips anyway, especially to the guy who gave me the right answer (but deleted it), I just didn't understand your comments.