I use Dropbox to sync across my devices using a free-tier Dropbox account.
On PC I use the official application, but since it has a 3 device limit, I use Maestral for the rest of my devices.
For mobile, I don’t use the official application, I use Dropsync since it allows to sync directly into the file system.
I have no idea if there’s something similar in iOS, but for Android devices it works like a charm. I have around 4 vaults synced this way.