Thank you so much for playing and recording the game! Loved your commentary and watching you play ATOMICA. Admittedly, this game doesn't do a top notch job of explaining how to play it, so here's some possibly helpful tips:
For a high score, the most I've seen is at a local playtest of around 18,000 and the playtester didn't really know how they got it (which seems to be a recurring theme in this experience haha). From what I can gather, the playtester played defensively once they got to a lower "life" (atomic stability) and used the gravity to bunch up the purple cubes to protect the core. As the cubes give a score on each hit with the yellow ball, the player managed to get quite a numbers boost as the lower the atomic stability, the higher the points earned (up to a maximum of 200 per hit). As such, at 10% stability or less, it is possible to get one large points boost before the explosion which can push the total up to an S.
The next hotfix will add in more tooltips and UI to help show scoring + how the extra points work based off the atomic stability (: