I want to second Dokuwiki. It is a step up from text files in a folder, easier to install and maintain than Jekyll (I am a big fan of Jekyll but have a deep-seated hate for Ruby). When compared to all the other wikis it is by far the easiest to install, maintain, and move to another server.
Dokuwiki is all text files and a version of markdown, the files can easily move to another markdown system.
First, you used the word client, so these people are going to be paying, the other is multiple Terrabytes over time. You want to set them up with something that is capable of growth. If you are not going with raw file storage (folders, labeling, organization) then what you are looking for is a DAM solution. Digital Asset Management. It takes a lot more work to put images into the system but they are designed to allow you to find, and share out digital media files.
My Goto Would be ResourceSpace (resourcespace.com) you can self-host it or pay them for cloud hosting. Do not do this on a nas. Use a real server with a Real VM so you can restore and migrate to newer hardware over time.
Finally once setup you are going to have to setup a backup solution that follows 3-2-1 backup.
By domain its actually the people who control the domain .us domain information not cloudflare. Cloudflare has been my goto for a number of years they do a good job masking the information and once you set those settings they do not change them (I am looking at you GoDaddy). I will second Porkbuna nd Namecheap as good options.