How can I fully teardown the keyboard (removing the pcb from the case)

I have not seen any that could not be separated from both top & bottom. You just need to find the screws holding the PCB to the top.

When I disassembled my G915, I could never get the front part off. It looked some of the buttons had a different design than the others. They appeard to be mounted from the top and soldered to the board, locking the top in place. I may of course have missed something, but there were NO more screws to remove and the top was firmly in place all across the board (not just in one place).

1. Remove all the keycaps.
2. Remove all the top screws size M2x6 (self tapping) with a ifixit PH1 Philips bit.
3. Remove all the M2.5x5 screws from back starting with corners with PH1 bit
4. Lift the sticker and use PH0 bit for the two screws M2.6 (custom self tapping with flat instead of V profile)
5. Remove the left and bottom feet and use PH1 bit to remove two M2.5x4 screws
6. Gently remove the battery from the connector and set aside
7. Remove the two M2.5x3 screws with PH1 bit
8. Back panel is free and the PCB with switches are on the front shell
9. You require a desoldering gun for the next step as all the switches are welded to the PCB through the front metal shell.
10. These traces are extremely tiny unlike other G series keyboards and solder flows through the through holes till the both ends of the PCB. Upon careful inspection if all the switches are properly desoldered, slightly pry the PCB. There would be a few weak solder joints that are 5% bonded between switch and PCB. PCB would easily lift off after that.