Yes, in the end we didn't have time to add more levels and we decided to deliver a "finished" small product rather than adding more and more and forget something else.
Regarding the fuel, we didn't have time for fine tuning yet and if we add more you would be able to reach the planets without needing to use their gravity, which would miss the whole game mechanic. Anyway, we are aware that it is rather difficult as it is right now and we may iterate it further after the jam!
Thank you very much for your nice feedback! :)