I routinely use Android for my phone and tablet.
I can't remember if I set it, or whether it is default, but if there is a WiFi connection, the things automatically use that in preference to the roaming/mobile data connection.
THis sometimes causes issues when travelling, as the wifi will try to connect to a shop or other open connection. Because it has connected to the wifi, but not logged in, there is no data transfer.
The work-around I use is to switch wifi off when travelling, or on the move, to put all the data over the SIM card.
When I specifically connect to wifi (at home, or in a hotel or coffee store), I the wifi on and switch mobile data off.
I also subscribe to a VPN and when using public wifi, engage the VPN, just to ensure a bit of data security.
It's not really any hassle and gives peace of mind
As to storing images on the micro SD, there are (free) apps that allow you to manage files and folders. I used to use ES File Explorer, but had some issues with that being a bit too intrusive and setting off alarms when I told it not to.
Just as a tip, I tend to try out three or four different Apps for any given function. You download one, set it up, give it all the permissions and see if it works for you. If not, delete it, and try another.
I'll probably try File Manager* next - look them up on the Android store.
*Other Apps are available