Here's my guide for gold stars, which is 800k and 100%. This is the version with commentary, explaining what I'm doing and generally why I'm doing it. I've also uploaded versions without commentary, which should be linked on the video page.
On hard, enemies shoot more often and deal double damage. On Journey, there's almost no difference between Normal and Hard outside of this. You may need to use a euphoria, but that's hardly an issue here.
The most important thing to remember is to keep your perfect multiplier going! The multiplier only counts for octa-locks, meaning you'll get minimal points for spamming. For gold stars, you'll want a combination of octalocks and spamming, but if you're not going for gold stars, it's best to always try octalocks, even if the enemy is about to get away. Your purification percentage takes the hit, but you get a few more points out of it.
Passion's probably the hardest to deal with. Get a x8 multiplier off the cores in the very beginning, it'll help boost early. The satellite will probably take the most practice to master; if you take too long, it starts shooting at you and you lose the 100%. If you're going for gold stars, take time to line up octalocks, but if you take long, you may want to spam. Getting that sense of timing just takes practice, unfortunately.
Points are readily abundant on Journey, and Hard does nothing to stop that. I had to use a euphoria in a moment of panic and still managed to breeze by the goal.
Also, if you're having trouble with octalocks, turn down the sound effects in the main menu. I dropped it to about 20% and the beat was very clear for me.