A challenge of which cloud provider to use will come up. Some in this thread mention OneDrive, I generally use Google Drive, and many others will use Dropbox, etc. To support them all will be a mammoth task. To support just 1 will only serve a subset.
Personally, I use Syncthing, which works great for me, but is a bit more complex than the more common options. Additionally, there is no native iOS client (I’m not an iOS user), due to iOS limited capability to share Syncthing files to another app that isn’t the Files app.
Perhaps to get more at the root of the problem. Why is the existing local folders which can then be synced by the OndDrive, Dropbox, Syncthing, clients not good enough? How can that be improved?
If LogSeq is to provide a cloud sync, I think it should be their own, rather than a particular existing one. Another app, Moo.do (renamed to LegendApp) started by utilising Google Drive as their backend storage. They then migrated to Firebase due to issues in doing what they needed to do. For the long term view are those generic fileshare options the right ones? What if you want to do pub/sub for notifications? What if you want to do automations on schedules without needing LogSeq running somewhere?
Just some thoughts. I’m not trying to dissuade cloud sync here, but raise how it can be a challenging choice.