What does syncing do? My samsung galaxy s4 mini offers two syncing methods : Samsung syncing and Google syncing.
My phone has important notes in the standard built-in app "S memo" and important recordings in the built-in voice recorder.
Does any of these two syncing methods sync this stuff ? Under "app syncing" maybe ?
If so, does that truly mean I can retrieve it if I lose my phone and buy a new Samsung (resp Android) phone ?
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.