The easiest solution is probably to outright remove the offending kernel extension from /System/Library/Extensions, in your case AMDRadeonX3000.kext. You need to:
Disable System Integrity Protection. There are many guides across the internet on how to do this, but the short version is: reboot into recovery mode, open the terminal, and enter
Copy /System/Library/Extensions/AMDRadeonX3000.kext to a safe space somewhere else on your computer, in case something goes wrong and you need to revert.
Delete AMDRadeonX3000.kext from /System/Library/Extensions.
Clear your kext cache. Open the terminal and run:
sudo touch /System/Library/Extensions && sudo kextcache -u /. Then reboot.
The kext may reappear after updating macOS, in which case you'll need to repeat steps 3 & 4. For this reason, although you can re-enable SIP once the kext has been removed, I recommend just leaving it off.
The Hackintosh community has more persistent methods of disabling specific kernel extensions. If you'd like, you could look into installing the Clover bootloader, which is targeted at Hackintosh users but should work on official hardware as well. For your needs, however, I think simply deleting the kext is the best solution.
Edit: This method will cause the dedicated GPU to continue receiving power, wasting the battery. In order to stop the GPU from receiving power, you will need to install Clover and then create a custom SSDT that switches off the GPU.