I'm finding the best approach is to only upgrade a few strategically located provinces and leave all other provinces at either first or second level (i.e. as i've found them, i never actually upgrade them) with a variety of markets, shrines and tea-houses, as appropriate to the location. These less developed provinces almost never have any troop production facilities (apart from the odd siege engineer, or occasionally to take advantage of unit bonuses). Markets are never upgraded past the first level.
Thus i have a small number of fully upgraded provinces each of which are capable of producing all varieties of units, at high skill levels (ideally located on armourer/weaponsmith or other bonus giving locations). These elite troops are then padded out with utterly mundane zero skill yari and bow ashigaru recruited from the more numerous underdeveloped provinces.
This lets you get massive growth in the early game (which is when it really matters) by having lots of surplus food, which you can keep going all the way through the mid game until you hit the realm divide event. At this point its probably worth sacrificing that extra food and upgrading your markets to boost your cash now that youve lost a lot of trade income.
It goes without saying that farms are upgraded everywhere as a priority, though i try to restrain myself to building the highest level farms at only fertile & very fertile locations.
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