Hi! I'm new to MonoGame, I experimented with XNA long ago though. I have made a few games in GameMaker Studio but I am now looking into moving to MonoGame.
To make content handling more convenient, I decided to create a tool that automatically adds assets to the Content.mgcb file. It is quite limited at the moment, but basically it adds supported files (also in subfolders) to the Content.mgcb file, ignoring files that already exist in the content file.
You just need to have a copy of the tool's exe file in the same folder as the content file and run it.
Like I said, I'm new to MonoGame so I'm not aware of the inside outs of content handling. Try the tool out and give me suggestions for improvement (at the moment it is more like a proof-of-concept). Not all asset types are supported yet, and support needs to be added for custom importers etc.
It's available on GitHub: https://github.com/MickGyver/MonoGameContentCrawler