I fit and sell the Spesh foot beds. If you really have a flat foot while standing, the high arch (+++ Green) foot beds would not be my first suggestion. When I have people try them on, I will usually pick the height that might be one size too high based on my visual analysis of their arch.
I do this especially if they have had insoles with minimal support, so they can see a big swing in difference of in-shoe feel. Then I will step down one size and retry. I let them choose, based on feel when pedaling.
But my analogy is that you should not feel a massive push (like a golf ball under your arch) from the support. Having that will lead to excessive pressure in a bad spot that creates a new problem in many cases, vs solving the lack of support on the other extreme.
From your comments, I’d think you should at least step down to the medium (++ Blue) size. Even that could be too high, but is worth a test vs dropping to the low (+ Red) size.