5M sodium ethoxide/methoxide solutions aren't solutions anymore; they are some kind of slurry. Ever tried to stir a slurry? Ever tried to dissolve a workable amount (say, 10 g.) of bromovanillin in a slurry like that?
Regarding the overoxidation: UTFSE, and yes, O2 is the main culprit. When you work on a small scale, it's easy to end up with a nice amount of benzoic acid instead of your aldehyde. You also have some fair chances of fucking things up during workup (at least when your solutions are still basic).
Now, if you want to work with 3,4m5-trimethoxybenzoic acid, that's easy to get. No need to do a 3-step reaction to get that.