So any other difference between the TD-8840 and TD8840T other than chipset?
The hardware in a modem/router is generally a chipset (motherboard, cpu, network controller, modem) + ram, nvram, flash ram etc
The non T uses a broadcom chipset, the T is a newer version using a Trendnet chipset. For whatever reason TP Link changed chipset vendor.
Typically the chipset vender supplies a base or reference drivers/firmware that the companies like TP Link, Netcomm (Dynalink), Linksys etc then customise or use to create the software that runs the device.
Broadcom DSL chipsets seem to have the least issues with the line cards in Chorus cabinets/exchanges.
For whatever reason the 8840T does not have a half bridge option, in it's software.