Please forgive me in advance for criticizing your idea, but it would work better if you painted the mirror with flat white paint.
Mirrors are actually quite bad at reflecting light that is useable to plants, at least when compared to mylar or flat white paint. They are made to reflect the spectrum of light that humans see. The light also has to pass through glass twice. It goes through the clear glass, hits the reflective back of the mirror, and passes back through the clear glass again, losing intensity each time. Cheaply made mirrors also have a lot of flaws that are not easy for the human eye to see, and you lose more light with each minor flaw.
If you want more light, paint everything flat white. Every dark surface in the greenhouse will absorb light, while white will reflect it.
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