Hello,
If you go trough this Laptop/Dell - ArchWiki the recommendation is to install the nvidia-390xx from AUR and if you look at the package page, it has the bumblebee-forceunload as optional. That means that in some case actually you have to disable/blacklist bumblebee to have it working properly.
In your case you can use the
or probably best is to install nvidia-390xx-dkms