Totally agree, education games in general are hard without spending loads of effort just to teach players the underlying concepts, which takes a lot out of the "playing" part of the game.
I did want to have a 'how much more speed is required' indicator of some kind, but had to go with the simple progress bar at the top due to time constraints, which doesn't work super well for numerous reasons. I do think the trajectory indicator you mentioned would be the best option since it would give players an intuitive understanding of how an orbit works in the first place, but I don't know if I have the required physics expertise for such a task. I will definitely try it later though!
Also, I had never thought about just giving the player an orbital speed requirement based on their altitude, this probably would've been a nice and simple feature to add if I had thought of it during the jam. :)
Anyhow, thanks so much for the feedback! I'm glad to see you were able to beat it, and I'll make sure to give your game a shot too!