PopOS is currently using modified GNOME on Xorg. It’s impossible to get mixed refresh rates on Xorg/X11 (which is the legacy display protocol) and with your setup you are pretty much stuck with Wayland (the modern display protocol still, still progressing as a platform) - which is what you tried first if you used Nobara, whether it’s KDE or GNOME.
Note that PopOS 24.04 (that will be released this fall iirc) will in fact run on Wayland with all new Cosmic desktop (it’s first full DE written from scratch since like 90s) and promises great NVIDIA support - which can definitely be the case given recent updates.
Now on the flickering issues that you experienced, they’re specific to the NVIDIA driver and are just being ironed out. There is the new explicit sync Wayland protocol, new NVIDIA driver, patches for XWayland, patches for Mesa, maybe something more. It still might require pulling something that didn’t make it to stable distro repositories, but I think Nobara provides that and for sure will when 40 will get released soon-ish. I don’t have NVIDIA GPU, but I saw conversations on Nobara Discord and they help each other get NVIDIA going so maybe ask there.
The time frame is a bit of a problem here. If you want to avoid tinkering, hold for a little longer and in few months most distros (that ship a Wayland session) will most likely just work with your setup. If you want it now, feel free to get your hands dirty and find a way to run NVIDIA on Wayland with explicit sync support.