Ah okay, thank you.
So I should not install nodejs via pamac, when I am using nvm, because that could cause conflicts, but the LTS version used as dependencies should be fine.
The version of node that is used in the terminal (node -v), is the the version set with nvm.
The --assume-installed nodejs=<version> is something I would use after a installation via “add or remove software” GUI fails. But I have to do that via CLI, there is no setting in the GUI nor would it make sense to add it to pacman.conf?
Edit:
Thanks for link to that other topic, however it caused some confusion for me again 
However, I’d recommend to use the nodejs package provided by the repository rather than the “manually installed” one if possible.
So should I install the pacman version for node parallel to the nvm version or not? I definitely need nvm for the older versions and easy version switching.