After quite a bit of thought I have decided I love two ideas put forth here, so I shall start with trying them and then see if it is necessary to do any further editing.

I love the idea of slapping a law bonus on each level of the town centre. I can put the pop growth bonus here too, +2 for the first two levels, +1 for the third level and no bonus for the levels afterwards. I will add +5 law to the first 3 levels. I am not sure about the later levels; minus the extra growth the law boost should not be needed, but the AI might experience widespread revolting if the bonus is suddenly removed and it has not built up sufficient infrastructure to keep its people happy. So I think I may leave that bonus.

Moving each tech level of troops down a barracks is so simple and elegant; love it. I do wonder what to do with the last level of barracks; it will become useless.

I think I may boost the cost of the militia level units, say make levy hoplites the same cost as normal hoplites to build but not in upkeep. That makes them more of a human town guard type unit than an AI mass production item, or at least so I hope.

I'm also going to fiddle about with build times to get the higher tech items in place faster; I remember what a massive improving effect this had on MTW.

Any further editing, such as my original plans to make most good units cheaper, can wait.