I liked it. Seems like if you want to expand it though you'll have to add more complexity. I get a very "The Witness" vibe to this game, so maybe fitting with that theme you could add some puzzles that are affected by the environment in some way? You could also add some rune that splits the beam into two, maybe have some runes that have different effects based on which way the beam is traveling so that in the end you can feel like a wizard weaving different runes to control two beams with one set of instructions.
Also I liked that at the end you end up having to write an actual word, and think that would be a pretty cool idea for a puzzle. Like you're stuck trying to do this insanely complex series of tasks and every time you do one thing it breaks another, only to realize that the solution was right in front of you the entire time. Sort of like Gandalf's "Speak friend and enter" moment. Some players would probably derive the solution without ever realizing you spelt out an actual word, so it could be a cool moment regardless.