I don't have Birks, but wear NAOTS, basically the same as Birks, though fit my feet better.
I have had several NAOTS of the original style as well as the removable footbeds. My experience has been that the removable footbeds do not give the same support as the original style. Because of that, I have had custom orthotic removable footbeds made for those sandals. They come closer to the original non-removable types in feel and support. And being that they are custom, certainly better for my feet!
If you are having problems with your feet, you might be better to see a foot specialist and ensure what you are wearing are the best you can do for your feet. I hesitated ... and have never regretted it.