Design Files
All my design files are available here: https://github.com/nanikgeorge/FeaturingYouSAO
Circuit
The core of this SAO is a pretty simple circuit - it's just an astable-multivibrator, pretty much the first thing that pops up when you google "simple oscillator". You can read more about it here: https://www.electronics-tutorials.ws/waveforms/astable.html
I then took the output from each side of the oscillator and buffered it with another BJT for each side - in hindsight this wasn't necessary, I overestimated how much current the LEDs needed. The BJTs I used to buffer also act as current sources that are set by the 5.6K resistors. They limit the current through the LEDs.
Things I Would Change
The LEDs ended up a little bright - I will probably swap the 5.6K resistors for 10K resistors next time I order them (if I order more). It's not so bright that I feel the need to make the fix to the copies I have - but something I will keep in mind for the future.
I'm guessing I'll think of some more things to change the more people I show it to and feedback I get, but that's all for now!