I think there might be a little bit of what is sound privacy advice being mixed up with what is sound security advice. The two are linked but some fall squarely into one category and not the other. I'm by no means an expert on the matter but my workplace did do a surprisingly detailed online privacy course ahead of my recent trip.
A VPN doesn't provide automatic privacy from snooping in and of itself, it is more like using another internet provider at another location (this could be in your current country or another). It can help with disguising your identity from internet monitors and sites you visit but you're placing your trust in the VPN provider and your browser can transmit other information that identifies you. In terms of protecting you against doxxing, it helps if IP addresses leak from a site that you have submitted content that a would-be doxxer deems worthy of doxxing. However, I believe it is quite rare for people to be doxxed purely from their IP addresses, as ISPs tend to regularly rotate IP addresses between users and it's more of an issue for government snooping. Similarly, the dark web only helps you hide where your information is coming from, it doesn't help hide the information you willingly put out there.
No matter how secure an email provider (e.g. Proton Mail) is, if you use an email address for both public and private communications (e.g. giving it on your personal website and using it to register for doxxable activities) then that will allow a doxxer to find you. A throwaway email address means that you can be locked out of a forum if you forget your password.
With all private forums, both those you set up and those you use, assume any information you put on them could be compromised. If the information isn't there, it can't be used against you. This is why I would disagree with the private Facebook group and Whatsapp suggestions, since those both require real-world identities to use, meaning an interloper can begin doxxing if they make their way in (which becomes exponentially more likely as the group size grows). If you're looking to build your own forum, the manual for phpBB (open source forum software) seems to suggest that, while it's not possible to completely disable the need for email addresses, it can be set up so that junk will be accepted and account activation is done manually with no email confirmation required.
Doxxing isn't unique to GC individuals. People doxx those who they believe have committed a social transgression or even purely for entertainment, they can do this for everything from a sense of justice to their own amusement. I'm not equating being GC with, for example, white supremacy, it can happen to anyone for any reason.