Really glad you enjoyed it!
I added a tips section to the page with a short video showing how you can get to the hard levels, if you want to give it another shot. Hopefully that will help, you aren't the the only person who has had trouble reaching them.
Regarding the rocket being slow, just in case: did you see the rocket tutorial panel in the hub (straight ahead when exiting the hard area) which explains that you can charge your shots so they fly faster?
Destroyable obstacles / buttons to open doors and a few other interaction ideas were on the list, but unfortunately we just didn't have time to explore them during the game jam. We do intend to revisit the game once the voting period is over though, quite a few things we want to try out!
Thanks for all the feedback, and hoping to see your times on the hard level leaderboards soon :)