I'm assuming that when you extend your leg with a flat foot that your leg is straight and/or you have to point your toe? When you extend your leg with a flat foot, you should have a slight bend to your knee. I'm assuming that you just need an inch or two to get you in a correct position.

I looked at the link & the seat looks pretty padded. Maybe a change there would get you lower. Also, if you add shoes & clips, rather than cages, that might help. Do you have any old platform shoes? Just kidding...

A problem with the seat being too high is that your hips rock side to side & eventually may give you some problems with your lower back. I have some spin students who were told many years ago that their legs should be straight & now they're talking about lower back pain. They have bad muscle memory, so anything set up correctly feels wrong to them.