GPU-defect Macbook Pro 2011 does not accept csrutil disable anymore (10.13.6)
I had same issue but did not have to use another computer or usb drive.
Via: https://apple.stackexchange.com/a/336676/302408

I was having the same issue. This is how I fixed it. 

As we are not able to get to single-user-recovery mode by holding Cmd+R+S at startup to run csrutil disable, it is not taking you to SingleUserRecoveryMode. 

I run the nvram command here in SingleUserMode

nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00

Then reboot

reboot

On reboot hold Cmd+R and it will take us to the GUI recovery mode. There click on Utility and open Terminal, here we can run the csrutil command.

csrutil disable

Then again make sure to run the nvram command then reboot

nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00

reboot

Boot into SingleUserMode Cmd+S to continue with the kext moving procedure. once done, go back to GUI recovery to re-enable SIP csrutil enable, then reboot once morew.

Once the Mac fully boots run nvram one last time as sudo

sudo nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00

Now do multiple reboots to test the machine boots back up properly. I have no issues now.