Hi Ben. I like the idea but i like to give you another idea. Don’t make something that works online. Make something that works on the desktop. A desktop CMS system.
In my younger years i worked a lot with Citydesk by Joel on Software.
I was a beta tester for Encor Pro from South Africa
I was a beta tester for an number of others also. I made for instance themes for Website Baker, a php based CMS system.
Wow a lot of memories come back now. I am talking about ten or fifteen years of (desktop) cms fun.
I like a desktop based CMS system a lot. You can work on it offline. You generate your pages as static html. For a lot of users this would simple be enough. No need for databases. With the build in ftp module component they can upload.
No need for commandline work for a the python and ruby based static html generators. I know you have experience programming in java. What if you used that?
I googled somewhat and found for instance https://getpublii.com/ they also have a windows and mac version but not a linux one. And i like Linux a lot. i just see they are maybe making one. https://publii.ticksy.com//ticket/1109971/
Surprised to see that they make publii with electron. https://electron.atom.io/ (i like electron based programms )
I stop my rambling now. Just trying to give you some ideas.
Good luck with whatever you are going to develop.