Grub GPU Switcher

custom.cfg

# copy the main menuentry from /boot/grub/grub.cfg and add it twice to /boot/grub/custom.cfg

# for Nvidia add the following to the end of the linux line: modprobe.blacklist=nouveau systemd.setenv=GPUMOD=nvidia rd.driver.blacklist=nouveau nouveau.modeset=0 nvidia.modeset=1
# for Nouveau add the following to the end of the linux line: modprobe.blacklist=nvidia systemd.setenv=GPUMOD=nouveau rd.driver.blacklist=nvidia nouveau.modeset=1 nvidia.modeset=0 
# for example (again, adjust for your system based on your main menu entry)

menuentry 'Manjaro Linux - Nvidia' --class manjaro --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-xxxxxxxx-xxxxxxx-xxxxxxx-xxxxx' {
	savedefault
	load_video
	set gfxpayload=keep
	insmod gzio
	insmod part_gpt
	insmod ext2
	search --no-floppy --fs-uuid --set=root xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx
	linux	/boot/vmlinuz-5.15-x86_64 root=UUID=xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx ro nowatchdog zswap.enabled=0 mitigations=off audit=0 quiet apparmor=1 security=apparmor resume=UUID=xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx udev.log_priority=3 modprobe.blacklist=nouveau systemd.setenv=GPUMOD=nvidia rd.driver.blacklist=nouveau nouveau.modeset=0 nvidia.modeset=1
	initrd	/boot/intel-ucode.img /boot/initramfs-5.15-x86_64.img
}
menuentry 'Manjaro Linux - Nouveau' --class manjaro --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx' {
	savedefault
	load_video
	set gfxpayload=keep
	insmod gzio
	insmod part_gpt
	insmod ext2
	search --no-floppy --fs-uuid --set=root xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx
	linux	/boot/vmlinuz-5.15-x86_64 root=UUID=xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx ro nowatchdog zswap.enabled=0 mitigations=off audit=0 quiet apparmor=1 security=apparmor resume=UUID=xxxxx-xxxxx-xxxxx-xxxxxx-xxxxxxx udev.log_priority=3 modprobe.blacklist=nvidia systemd.setenv=GPUMOD=nouveau rd.driver.blacklist=nvidia nouveau.modeset=1 nvidia.modeset=0 
	initrd	/boot/intel-ucode.img /boot/initramfs-5.15-x86_64.img
}

gpu-switcher

#!/bin/bash
#
if [[ "$GPUMOD" == "" ]] ; then
# uncomment nouveau blacklists
sed -i '/blacklist nouveau/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist ttm/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist drm_kms_helper/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist drm/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
cat /dev/null > /etc/X11/xorg.conf.d/01-nv.conf
elif [[ "$GPUMOD" == "nvidia" ]] ; then
# uncomment nouveau blacklists
sed -i '/blacklist nouveau/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist ttm/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist drm_kms_helper/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist drm/s/^#//g' "/etc/modprobe.d/mhwd-gpu.conf"
# for the following, you can find your specific gpu's Section Device using nvidia-settings gui
# https://wiki.archlinux.org/title/NVIDIA#nvidia-settings
cat > /etc/X11/xorg.conf.d/01-nv.conf << EOF
Section "Device"
  Identifier   "Device0"
  Driver       "nvidia"
  Option       "NoLogo" "True"
EndSection
EOF
elif [[ "$GPUMOD" == "nouveau" ]] ; then
# if you get "Failed to start virtualization daemon" error on boot, due to libvirtd service failing,
# uncomment the following systemctl to mask libvirtd:
# systemctl mask libvirtd
#
# comment nouveau blacklists
sed -i '/blacklist nouveau/s/^/#/g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist ttm/s/^/#/g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist drm_kms_helper/s/^/#/g' "/etc/modprobe.d/mhwd-gpu.conf"
sed -i '/blacklist drm/s/^/#/g' "/etc/modprobe.d/mhwd-gpu.conf"
cat > /etc/X11/xorg.conf.d/01-nv.conf << EOF
Section "Device"
  Identifier   "Device0"
  Driver       "nouveau"
EndSection
EOF
fi

gpu-switcher-info

Follow this article https://arashmilani.com/post?id=86
And rename My Script and /my-script to GPU Switcher and gpu-switcher, or something similar.

Then create /usr/bin/gpu-switcher and add the gpu-switcher code to it.

This has been adapted from the from the nvidia.start script found here:

https://wiki.gentoo.org/wiki/Nouveau_%26_nvidia-drivers_switching#Switching_using_a_single_kernel_and_hprofile

You may have to adjust each Section "Device" entry of that script for your drivers and system. 
I don't know what they should be exactly. 
It's possible you can find more info under some file in: /etc/X11/xorg.conf.d/ - or in the forums/google.

Once complete, you can select the custom grub menu entry for a driver of your choice on boot.
Note: to see grub menu, you may need to change the following values in /etc/default/grub for the menu to show up, 
if not already visable.

GRUB_DEFAULT=saved
GRUB_TIMEOUT=10
GRUB_TIMEOUT_STYLE=menu

GRUB_SAVEDEFAULT=true

Don't forget to sudo update-grub after changing this file.