I know it's possible to set authentication on all node-red endpoints via 'httpNodeauth'. But is there a way to set authentication on dashboard only?
No there isnt
As an ajunct to Dave's short and to-the-point answer 
You can do this via a reverse proxy. Please search for the various threads on this in the forum. Including some documentation that I wrote mainly for uibuilder users but that is generally applicable as well for configuring NGINX.
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