Thanks for all of your help. I think part of my confusion has come from trouble finding & interpreting information like that (e.g. lots of pages I was reading were old / archived from before the lede split & merge).
Also, I found that by following this article and using sysupgrade with a "stripped" factory image, I was able to restore the TP-link factory firmware to this device. Perhaps I should have mentioned that the reason I was working with this old device again is that I recently decommissioned it from production and wanted to put it to more casual use, hence returning to OEM stock firmware is an acceptable end-state. I look forward to trying the latest OpenWRT release on newer hardware versions of this device (love that the USB host makes it possible to overlay a much larger file system).