Phone decision help, used flagship

[pete6032]

Criteria
I am looking to spend $300 on a used phone on Sprint network.
I want to get a phone that supports Sprint's LTE plus network (tri-band carrier aggregation).

Given this price range and LTE plus requirement that basically limits me to the last round of flagship devices - S6, S6 Edge, LG G4, LG G-Flex 2, and HTC One m9. Out of my price range are the HTC ONE a9, Nexus 5X, Nexus 6P Note 5. Not supporting LTE Plus are any Motorola or previous generation nexus phones.

Needs
I don't ever play games on my phone so computing power is not a big consideration. My One M7 from 2.5 years ago still gets the job done for me.
I would like something that I can root easily and run something close to stock Android ROM on it (Concerned about the bloat of the S6, also concerned about S-off on the HTC, would I have to pay someone to get it S-off?)
Screen size is not a big deal, and I'm fine with either AMOLED or LCD.
Plan to use this new purchase for a couple of years.

What would you recommend? Thanks.

 

[Golfdriver97]

Most of your choices are going to be difficult to root. If I had to suggest one of those, it would probably be the M9.

 

[pete6032]

Most of your choices are going to be difficult to root. If I had to suggest one of those, it would probably be the M9.

I have not ever owned a Samsung phone. With HTC and LG I was able to figure out how to root based on guides posted on XDA. Is the Galaxy s6 more difficult to root? Because of locked bootloaders?

 

[Golfdriver97]

Because of locked bootloaders?

Depending on the carrier, yes.