Should I buy a seperate router to fix my wifi issues? I currently have a 5268AC

I've been having trouble with wifi.  I had an NVG589 and recently upgraded to 45mbps tier (highest available this far from the vrad) and got a 5268AC and pair bonding with the upgrade.

I'm trying to decide if getting a standalone router would help or not.  Looking at something like maybe a nighthawk r7000p or a linksys eat 7300, ea8500.

The 5268AC claims to have all the latest features I'd want in a router, but is it actually just cheap crap?  I mean it claims to have a 1700mbps 4x4 MU-MIMO (although I doubt any of my devices have MU)  and I think at best my laptops have 2x2 antennas.  Maybe it doesn't have enough ram?

The problem is my wifi is still pretty bad.  My house is only 1600 sq. feet, although it is constructed with concrete block and its suburban with only a few neighbors networks even visible at all.  The biggest problem is in one corner of the house, and the backyard. The router itself is located almost exactly in the center of the house.  With the NVG 589 I had some service in most of backyard, with 5268AC it drops as soon as I step outside.  In the bad corner of the house, I'll get anywhere from 1-4 bars, its very inconsistent.  Also if this helps give any clues as to my problems, I have a vap2500 for the 2 wireless stb's and in one room it's always 4-5 bars, but the other room that is in the bad corner, it can vary wildly from 1-4,  although strangely it never seems to have picture issues even at 1 bar, although it does sometimes have connection issues with dvr and requires a restart.

Another one of my issues is streaming 4k, for instance technically it works, but it takes a long time to get started.  If I start a 4k netflix stream, it will start at 480p and take about 5 minutes to work it's way up to 4k, and then will be fine with no buffering.  Oddly the netflix app reports the tv is getting 30mbps, but the tv settings speedtest at the exact same time will report it is only getting 8mbps.

When I run speedtests they aren't even that bad often 30-40mbps, even in the furthest room, but there doesn't seem to be any consistency.  And if I try to play an online game I lag like crazy, I'm lucky to get under 150ms on pubg mobile and end up switching to verizon LTE if I have even 2 bars of LTE service it's better.  

I also have a number of devices, and wonder if maybe it's just too many connections for the 5268AC to handle?  Although most of them are using relatively little data.  3 laptops, an ipad, 3 smartphones, nest thermostat, 3 smart tvs, 2 kindle fire tv sticks, an amazon echo, a wireless printer.  

So is my issue the router? or at&t's network?  I don't want to deal with an extender and jumping between networks, so is my only option going all the way up to a mesh network, which seems like overkill for a house of this size.

What would you do?