I thing you are mostly right. However, there is a few complications.
Trade guilds might be worth it if the town has particularly high wealth (trade goods + decent fertility) and will have a metsuke to watch over it in any case - this would be regardless of the food effect. The food reduces growth in all provinces, but that loss of wealth is taxed with 20%. The growth boost + wealth addition in the "wealthy" town gets taxed at 35%+
It is not uncommon to get more than 2k of income out of the towns where I have maxed wealth and Metsuke sitting in them. Since you get 5 of them, the contribution is 10k+ to the whole wealth.
This effect should not be ignored since I find that often your tax is something like 40 - 50% from your top 5 income provinces when you might have 20-25 provinces in total.
I am not sure if there is an easy way to calculate these things, but I would start by assuming 5 metsuke of around 5 stars average in the overseeing towns skill, sitting in the top 5 wealth provinces (usually these would have from 3-6k wealth each depending on how far the game has gotten).
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