For completeness the answer will also include everything some of which you already know.
I understand in your situation that the phone is not distinguishing from your hotspot style of Wi-Fi (using expensive data from a cell tower) to any other Wi-Fi connection that the data would be cheaper to use.
Manual and automatic methods
There are methods to both easily manually disable Auto-Sync, and methods to automate the disabling of it:
- If connected to "Home WiFi", then Turn on Auto-Sync (less account data).
- If connected to "TeatherName WiFi", then Turn off Auto-Sync
Simple "scripters" like "MacroDroid", plus other simple "rules creators" can pull this off.
I mention these 2 because I could not program my way out of a paper bag, but I can make a few scripts, or rules, or constraints, using those methods. Probably tasker and automate-it could also perform these functions. Auto-Sync off is a "easy one".
In my short time with that Macrodroid app, I was also able to have it reliably recognise that specific Bluetooth devices are connected (keyboard), so that could also be applied to make a auto-sync change.
It can also use a "hotspot" trigger and actions , which won't do any good in your situation, because the hotspot isn't the device you need to control. That method would have to be applied to the hotspot machine, and some creative firewalling, that does not sound so easy.
Stopping Google Store auto updates Manually
To access the setting when your in the Google Play store application hit your menu button, and select settings.
Go to the Auto-Update section of the settings. Setting your apps to never auto update does mean that you have to manually update them, but that gives you the option to choose when the updates occur, giving you control,
The google play store settings for update applications can be set to update as follows:
A) Do not auto-update apps
B) Auto Update at any time
C) Auto update apps over Wi-Fi only
Manual updates at the google store are made easy
When you select the play store icon, and go to My Apps section.
The google store sorts apps that have updates available to the top of the "My Apps" listing
By carefully adding updates to your device manually you can know if a specific update is causing problems.
Widget Auto-Sync switch
There are many "widget" programs that will add an icon on your home screen to disable Auto Sync easily.
Widget Manual Sync Starting
There are also Widget apps that have "Sync Now" button, with SyncNow syncing will be updated manually at the press of a button. On my machine this activity ends, after any needed sync is done.
Background Data from Apps
In the network settings you click on the mobile data
Clicking on the menu button to get to the settings.
We can see the Restrict background setting, and the auto-sync data setting.
Automatic , Identifying a mobile hotspot for no background app data
In the above settings Mobile data / menu button / Mobile Hotspots
In the Mobile hotspot settings , you can select exisiting mobile hotspots , to restrict background app data.