This makes some sense… but not from a stiffness perspective.
The shoe isn’t shaped for your foot… or rather that size of that shoe isn’t shaped for your foot. These shoes don’t have any padding/give like you’d have in any other type of shoe. Keeping in mind these are 3d shapes, you can’t size the shoe in a normal way. I’ll bet you’d have the same issue in that stiff shoe without the insoles in, with the shoes full loose, sitting at home, working from a desk or watching TV. Try this before committing to a new shoe
I had a similar issue. Moving to a larger size put the arch more forward, in a better spot, fixing the whole problem. Maybe, on the same theme try an insole with a big forefoot button (Sole brand) or loosening up your shoe.
If that theme doesn’t work, try a different brand that is flatter or more curved, buy a few of them, and repeat the fully loose experiment mentioned above.