Thanks a lot, this is exactly the feedback I needed. You're right: after the first stage the runs feel too similar, and the pulsars and rivals barely change how you play.
The next update is all about that. Every stage gets a mechanic that changes how you move: rotating pulsar beams to dodge, rivals that grow, hunt you and are actually worth eating, stars that blow up into supernovas, meteor showers and orbiting moons. Bosses will behave differently too. And I won't stretch the game by making upgrades more expensive.
I'll post a devlog when it's out. Would be great if you gave it another try then!