For reference for anyone curious, my personal method for shmups in general, and it indeed helped me massively here, is research.
I have written down the order of enemies from the start to the end of their loop. I have rated waves for their speed, and overall difficulty. Knowing when things are coming takes the guess work out of it, and helps massively. Since the enemies ALWAYS come in the same order, I always knows what's coming up and can be prepared ahead of time, no guessing at all.
Also, the ridiculous high score is because of the constraints of this contest. The most points you can possibly get is from the terrain. The 80k bonus rows, and the Cleopatra in particular are huge. I got 3 Cleos, each worth 1 million, that's half my score right there.
So rather than focusing on killing stuff, I focus on dragging out as many enemy waves as possible, so I can loop the terrain as far as I can. Less enemy waves means easier enemies overall. Regardless, you will have to fully get past every single enemy wave to beat the game, and it's incredibly difficult to do on one life.
The best advice I can give is simply "watch your ship". If you're concerned about what the enemies are doing, you'll get hit by a stray bullet. Watch your ship and find a path through the bullets.
-------------------------