What does syncing do?
I can't speak to the apps your using but in general I'll audit/limit network traffic with a firewall just to be safe. As you've not specified if root or not I'll suggest two that I've had good experiences with.

NoRoot Firewall uses a neat trick with Android VPN built in options to allow for both logging and blocking network access; per interface (cell antenna or WiFi) and per application.

AFWall (root required) uses iptables and a fancy user interface to allow for even finer grain control.

Do be aware that blocking apps that require network access, such as a browser, will break the apps' features until unblocked. But using either will allow for you to find out exactly to whom each app is talking to in each network state... you can even catch dumb apps trying to make connections when in airplane mode.