You can certainly get to Mugello by train and is actually the best option, for the reasons you already know -too much traffic otherwise.
Check schedules on
www.trenitalia.com - for fast travel the morning of, the best option might be to take a fast train
from Venice to Bologna and then take a train to
Borgo San Lorenzo. That's the name of the town closest to the race track with a train station (the track is just outside the town of Scarperia); from Borgo San Lorenzo, there will be bus shuttles that take people to the race track.
Are you planning to return to Venice the same day? Once you check the train schedules you'll be better able to plan whether that will be doable or too tiring to fit travel back at the end of the day.
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