Thanks for playing! You can hold 'Space' to keep the white line as long as you want! (though, you have to dash when you let go of Space..). When holding space, none of your other movement options are restricted either, so you can ascend/descend freely.
Keeping the player distinct while keeping the artistic vision in tact was a struggle throughout the entire development process. It's not perfect, but it's a lot better than it was early in development.