I believe the Moto Z2 Force has excellent RF performance. It's also pretty cheap (well under $300 new or like new), has a Snapdragon 835 (like the Essential), near-stock Android (will be updated to Pie), and excellent battery life. I believe it also supports HPUE and Massive MIMO, and will also support wifi calling. Hopefully at some point it will support VoLTE.
The one downside to the Z2 Force is the shatterproof screen. It's got a plastic top layer that is prone to scratches. Motorola will replace it for you, but I'd suggest getting a tempered-glass screen protector as soon as you get it. The tempered glass feels much better than the plastic and will protect the plastic from scratching, while maintaining the shatterproof glass.