So I understand that the graphics module comes with an AMD Radeon RX 7700S. What type of GPU can I replace it with? Is it just a certain form factor of desktop GPUs, or what? Is there a limit for what type of GPU I should put in there due to the battery or charger?
No, you completely misunderstood what the module is. It contains a custom laptop grade GPU, it is not an adaptor in any way and you cannot just pop the 7700s out and put something else in.
However, the expansion bay can house a different GPU. Framework is supposed to release more later on and 3rd parties might as well.
The bay is limited to 100W of power draw, but theoretically, you could design and adaptor expansion bay able to house a full desktop grade GPU, provided you gave it some kind of external power supply