To sync your important data such as contacts, calendars, photos, videos, documents, etc., a server is required.
Instead of Mac or Windows, which are not open source, it is recommended to switch to a Linux system like Manjaro, Arch, Ubuntu and so on.
The smartphone is the biggest data leak problem that should be fixed as soon as possible. So if you have an Android device, you should solve this problem first. The server installation is recommended in combination with the smartphone in order not to lose the full functionality of many applications like the cloud feature.