Sorry if I misunderstood - and i get what you mean better now. As far as I know - the answer is no… unlike with many text editors, you cannot collapse sections of code within a cell. Not a bad idea though!
Not that this really gets you where you are going, but it is my understanding that if you have a lot of functions in a single large code block, you can (and should) try to break it down so that each function is a cell unto itself. Breaking things down in such a way would effectively give you the sort of collapsing capacity that you’re after.
Here’s the idea (fork of your notebook)… though again… sorry that I didn’t get you all the way(and really cannot… as I am very new to all of this and hardly understand anything):