The full moon is about 31 arc minutes in apparent size. Andromeda is about 190 arc minutes in apparent size. Based on my Eclipse photos at 700mm, the biggest issue you’re likely to have with the 300 f/2.8 is picking what part of Andromeda you want to fit in your photo.
You’re not an exempt (salaried) employee if they deduct your pay for working less in a given week. I’ve never had an employer who cared about hours as long as work got done.