I'm in over my head with solving my company's network

So I wrote about this about 2 months ago on /r/sysadmin but I haven't really been able to resolve anything.

As a backstory, my company has 50+ branches nationwide, about 5 "home" offices and the 1 corporate office. Right now, 3 of our offices are linked with tunnel VPNs through SonicWall devices, 2 others are linked in a central state from an acquisition but that's it. Every where else is fractured.

For the business, this doesn't mean much. Most of our employees work via email and web applications (in house dev and 3rd party). They don't notice that they can't connect to the file server, as an example, because they don't need it 90% of the time. They have little to no need with sharing data with each other as each branch basically operates their territory independently. Also the most data they share with the home offices is email attachments (although that is a process I want to change).

But for IT purposes every single user and PC becomes a black hole for me. Yes I can support them with tools like Splashtop but I can't do the bigger, better stuff I want to do. I can't deploy group policies, or software, or manage windows update or maintain assets. Or print servers or any of that stuff. So I need a network solution.

But I'm not a network engineer but I need a solution, I'm a do-it-all IT guy and this way out of my depth of field. In the last few months I've been reading about different ways to tackle this situation and I just don't know what to do.

Yesterday I was reading about ADVPN with FortiGates and thinking that might be a decent solution. A few weeks ago I was messing around with OSPF on my SonicWalls and thinking that might work for us. But again, I'm out of my depth here and I just don't know what's going to work. I'm now exploring an SD-WAN option with a broker we work with but based off a post I read here 50+ sites might be budget breaking.

Can anyone here help me that if I just want to manage our branches and home offices from an IT perspective what the simplest solution should be?