i think there is, but i dont think optimus-manager is the way to go. finding a xorg configuration that works first is probably a better idea, then setup optimus-switch thats much easier to bend to your will.
are you able to make it to a graphical desktop on any manjaro? live? installed? and if so what did you do to get there.
or if you can chroot in from another distro's live usb and post these outputs
inxi -Fxxxza mhwd -li ls -laR /etc/X11 ; cat /etc/X11/xorg.conf.d/*.conf ls -la /etc/modprobe.d ; cat /etc/modprobe.d/*.conf ls -la /etc/modules-load.d ; cat /etc/modules-load.d/*.conf
what you need is a properly setup xorg configuration, and the nvidia gpu should be able to offload to the amdgpu/modesetting driver.
do you have the option available in bios to disable one of the gpu's?