Why not use a common git server? Why not use a dedicated backup solution? Many backup apps can store backups to Dropbox if that's what you want.
Myself, when I'm at home, I just check out the project on my desktop, use SSHFS to mount it on my laptop, edit it on my laptop or desktop as needed, and have an SSH window open from my laptop to my desktop for doing things like running npm (because that'd be way too slow if I ran it from the laptop and depended on SSHFS to sync it.)
If X11 or RDP was snappier, I'd just use my laptop as a thin client, but it's not, so I'm stuck with figuring out multi-master realtime source-code tree sync.
A similar problem obtains when using VMs, but at least there, there isn't the requirement that I have the ability to run a GUI text editor in the VM that sees changes I make outside the VM quickly enough to be unnoticeable.
basically, use FUSE to create an alternative view of your filesytem, but that fuse layer will also exclude certain files matching a regex. Only the FUSE view into your files should put in the Dropbox folder and synced. This particular fuse setup is read-only which was fine for my purposes, but you might need an alternative which allows write access for Dropbox.
But for personal projects I use syncthing, which supports .stignore, gives you versioning options, and most importantly doesn't randomly revert your files back to a previous version because of clock drift.
There are options for not syncing some files, for how to resolve conflicts, etc.