This is probably old news to many but since some guide makers have misinterpreted this achievement's description, I also did before getting it just now, you don't need 10 large garages with at least 75% productivity, you need the AVERAGE of those 10 large garages to be above 75%.
Every single large garage that you have above 75%, for example 100%, still keeps contributing more to that average. I just unlocked this achievement with only 5 garages being above 75% but because 3 of them were 100%, the average triggered at 75.1% when the 10th large garage had 48% productivity. If every single percentage point above 75% didn't matter, my average would only be at 65.9% currently.