I wear a combination of hard soled boots (Meindl) that flex very little anywhere and offer good support - but you wouldn't want to wear them around town - well I suppose you could but hmmm... anyway, during the summer months I wear the more modern type of fabric walking mid-boots. They tend to be much lighter and do flex much more than the rigid boot.
The main area of flex - and they vary from make to make, is in the mid-foot. They also tend to flex much more in the toe area which I like but will be a problem for you, but they do differ greatly between manufacturers. I have a worrying tendency to trip when wearing my Meindl Burm Pro's with the clompy soles. There may be something in between, perhaps Brasher or HiTek might do something with a firmer sole but more casual.
As for the theory - there are those that extol the virtues of the more rigid boot and those promoting the more 'trainer' style of boot or even shoe. Really, it's a case of finding something that you like but whatever you do, make sure they are correctly fitted - too loose or too tight brings problems of their own!
Good luck. I'm sure you'll get plenty of feedback from these guys here