asks: Can a plot ever be too much? Like if you have lots of things planned to add extra drama/tension, could this end up taking away from the main plot, or overwhelm the reader? Because that's the last thing I want to do!

Think of a story as a series of overlapping arcs. You have the plot arc and character arcs. Stories that are action-driven may not have much of a character arc, but let's assume you have some character development. A character's development often corresponds with certain plot points. For example, when the hero finally discovers the confidence she needs to perform for the school, which impresses an important donor, who in turn saves the music school from closing.

There can also be secondary character arcs, where a side character undergoes some personal development as well. Maybe a best friend realizes they're more than just moral support. Maybe even a third character arc as well. More than that, however, the story might start to feel disjointed or convoluted. None of the secondary arcs should overshadow the main character's arc, though it can be extensive if that character will be featured in a sequel.

Now, to answer your question. Does the extra drama you want to add move any of the arcs forward? Does the main plot advance, or the main character learn something, or a secondary character grows in some way? If not, then maybe it's unimportant fluff that is distracting from the main points of the story.

I recently critiqued a friend's story where a picnic was detailed. The food and scenery was described. Even the eating was described. Then there was some fun water-splashing in a nearby stream. While all of it was cute, NONE of it moved any of the arcs forward. The plot was at a standstill while all this was going on. The characters experienced no growth. So even though all this happened as part of their quest travels, it was quite irrelevant to the story's bigger picture. I recommended the entire scene be cut because it had no purpose, and my friend reluctantly agreed.