Same problem here. It would be a game change if you can fix the problem. See my old thread for the same issue https://discuss.cachyos.org/t/could-not-get-hibernate-to-work/10130
After some debugging I think I found the root cause: CachyOS loads the Nvidia modules early into the initramfs via chwd (/etc/mkinitcpio.conf.d/10-chwd.conf adds nvidia nvidia_modeset nvidia_uvm nvidia_drm to MODULES), while at the same time nvidia-sleep.conf sets NVreg_PreserveVideoMemoryAllocations=1.
I thinkt the problem is that when the Nvidia driver is loaded this early during boot, the /proc/driver/nvidia/suspend interface isn’t ready yet. So when the kernel tries to resume, the driver fails to restore video memory and returns error -5, causing the resume to fail.
The nvidia-hibernate.service and nvidia-resume.service systemd services are designed to interact with this procfs interface, but I think they run too late — the driver has already been loaded and failed by that point.