As a D2 owner, to confirm a couple of things: My D2 originally shipped with Anduril version number 2021-12-13-0135 as well. Model number 0135 was originally named emisar-d4sv2-tintramp, and is now named emisar-2ch, cf. https://toykeeper.net/torches/fsm/anduril2/MODELS. Over there, it is also specified as attiny1634.

Scrolling down https://toykeeper.net/torches/fsm/anduril2/, you will find the latest emisar-2ch version at the bottom, currently indeed anduril.2023-08-07.emisar-2ch.hex. Over there, you will also still find anduril.2021-12-13.emisar-d4sv2-tintramp.hex.

For instructions specifically for backing up, cf. for example https://anduril.click/flashing/avrdude.html#backing-up.

Usually, I am using ZFlasher AVR via mobile phone (Android), which is very straightforward, but I just checked with avrdude (on Linux), and I had no difficulties with pinging the device, backing up, and updating, while using the instructions over at https://anduril.click/flashing/avrdude.html.

Finally, cf. https://bazaar.launchpad.net/~toykeeper/flashlight-firmware/multi-channel/view/head:/ToyKeeper/spaghetti-monster/anduril/hank-cfg.h to see which modifications Hank lights have from stock Anduril 2. Recently, a couple of functions were allowed in Simple UI, namely ramping toggle as well as aux config and strobe modes.