Yes, you can install the 5.4 kernel:
sudo apt install linux-headers-5.4.0-81-generic linux-modules-5.4.0-81-generic linux-modules-extra-5.4.0-81-generic linux-image-5.4.0-81-generic
You can use this method to lock the kernel in:
It's hard to say. But testing out both is reasonable and safe.
You can install the 5.4 kernel and test out how things work.
Then test Nvidia vs. Nouveau and decide which works better for you.
If you see no appreciable change, just stick with the current kernel.
If you do see one, lock in the older kernel.