Thank you for all the ideas and comments!
Many of your points are planned to be addressed in Version 3.0.
There are plans to add some "special stage" that will trigger when going into high altitudes which will provide some bonuses to the player (as long as they play skillfully), so there's less "dead time" while IRyS is off-screen.
There are plans to make downboost upgradeable to widen the aim angle and also give it more power since currently it's mainly conserves the current player velocity. However it will remain to be only usable while moving up. The main use is to force a landing on an item closer than the predicted landing spot, so you don't have to wait to see what's coming up later in the flight trajectory. Best example is downboosting to catch a soda right after Calli boosts you up or after manually boosting to avoid Bae. Other uses are to "chain" boosts, by quickly downboosting after a Calli into a Mumei. It's not recommended to downboost into a Kronii as the angle is not high enough to guarantee a horizontal boost (at least not until update 3.0)
A visual indicator for upcoming air items is planned for update 3.0 as well.