As above, it is not possible to be able to put both feet flat on the ground and get proper leg extension on a standard bike, regardless of frame size - your legs should be damn near completely straight when the pedal is at the bottom of its stroke, and there needs to be some clearance between the pedal and the ground. Feet flat on the ground when seated with proper leg extension on the pedals is a geometric impossibility.
Possible solutions:
A 'crank forward' bike as
Iride01 suggested. I don't know who makes these in 2021 - Giant and Electra?
A recumbent bike, which has the pedals out in front of the saddle instead of underneath it.
https://en.wikipedia.org/wiki/Recumbent_bicycle
A change in priorities for the rider - either practice sliding forward off the saddle when stopping and standing over the frame (the strategy most cyclists use), or leave the saddle lower than ideal but give up on longer rides.