As a rural customer, don't you have access to landline DSL service? Roughly $40 for DSL on top of your datacard service may seem like a lot, but wait until you go over your 5gb monthly data allowance. The overage charges can easily swamp that amount.
I think you're on the right track limiting pictures and video, but good luck enforcing this discipline on multiple users.
How about AV updates? How often are your AV security programs looking to update? Norton often syncs several times a day, almost constant live updating, by default, if you do not reconfigure it. Each time an AV syncs, there will be data exchange, even if there is no new update. Those files may only be a few K each. Other AV programs sync intervals are configurable and usually range from daily to weekly as a default.
Are your apps set to auto update?
Do you use a software updater to manage your third party programs or apps?
May God bless your travels
Me, The Wonderful Wife
and two Spastic Border Collies U.S. Army Retired 2013 Tiffin Allegro 36LA 2007 Saturn Outlook, FROG