Yet another: the 96Boards[1] DragonBoard 410c[2][3] from Qualcomm has a Snapdragon 410 quad-core Cortex-A53 with a good amount of connectivity and IO interfaces (as per the 96Boards spec) and an Adreno 306 GPU which supports the open source Freedreno[4] GPU drivers (if you have a binary blob allergy). That said, this thing has a whole different performance tier than the TX1, but if you wanted to start poking at 64-bit ARM, this thing might be worth a look, especially with the comparatively low price.

[1] https://www.96boards.org/

[2] https://www.96boards.org/products/ce/dragonboard410c/

[3] https://developer.qualcomm.com/hardware/dragonboard-410c

[4] https://freedreno.github.io/