When you want to use your nvidia card with nvidia drivers, you can install the kernel 5.15 LTS (which is an older kernel that supports the nvidia 340 driver) or add this ppa to your system:
ppa:kelebek333/nvidia-legacy
This ppa makes that you can use the nividia 340 driver with the normal kernel of Zorin 17. Then install nvidia 340 driver metapackage.
If you want to do some of this we can give you more detailed information.
Edit: If you have upgraded from Zorin 16, you can simply select the older 5.15 kernel in the grub boot menu at advanced options and set it as the default kernel. You do not need to reinstall it.