If you're looking for an open-source replacement, I have been really impressed with Syncthing. It is decentralised (and directly device-to-device), but if you can afford the space on your phone or an always-on computer at home, it is basically a drop-in replacement for Dropbox. It only worries about syncing files. Nothing else.

https://syncthing.net/

Or if you want Dropbox, because you need it for work or because it makes it's easier to share with family and friends, there is an awesome open source Dropbox client:

https://maestral.app

I have been running it for months now without any issues.

Thanks for that - looks quite promising.