I decided to try and package my game for sharing yesterday, and it was a rather huge file. I also noticed through testing it that the post processing on level 2 was ugly and wrong, and the game opened in a really strange way across my two monitors, with no way to snap it to a single monitor, drag it around or resize it - even though it wasn't set to cook for borderless windowed mode. I spent a while playing with the post processing until I got it looking good, then continued working on level 2, adding some new collectibles and a small path that leads to the next level. I also noticed that my score wasn't carrying over between levels, but that was a problem for later.
Today, I created level 3, a snowy, mountainous region with some ancient architecture. There's still a way to go with level design, but I've got a good amount of the main path figured out.
I'm happy that this is really feeling like a game now, and the level designs are really all I need to worry about unless for some reason I want to add enemies in the future.