I wanted to add a post here detailing my installation of OpenWrt onto an Archer A7. I ran into some issues not covered on the device page but was able to find a way to eventually get everything working. A user on /r/openwrt posted about having similar issues as me so I hope others will be able to find this post while troubleshooting.
To begin with, OpenWRT would not flash successfully over 2.1 versions of the TP-Link firmware using TFTP. My device came with firmware version 210125. I confirmed that my TFTP server was in fact functioning correctly by downloading the correctly named OpenWrt recovery file from my PC using TFTP on my Raspberry Pi. What ended up working was downloading the oldest available version of the TP-Link firmware off their site, 201029 at the time of writing, and flashing back the router using TFTP to the older version. I was then able to use the TP-Link GUI to install OpenWrt.
This installation path may become invalid if TP-Link removes the older versions of the firmware from their site. Maybe some users more in the know can chime in if there is a firmware archive outside of the OEM device download page.
I hope this helps if anyone else runs into issues!