I decided to stop pretending I will ever finish this project, and put it out of its misery.

I still believe it's generally possible to get the exact hardware I used to display animated GIF that you can easily upload to an USB drive. However, it would require a bit of work to write the firmware that does it, and I find that I no longer am interested in doing that work.

While I was procrastinating, CircuitPython grew a gifio library, that displays animated GIFs straight from the filesystem. It's as simple to use as:

import board
import gifio
import displayio
import time
import struct


display = board.DISPLAY
odg = gifio.OnDiskGif('sample.gif')

start = time.monotonic()
next_delay = odg.next_frame() # Load the first frame
end = time.monotonic()
overhead = end - start
display.auto_refresh = False
display_bus = display.bus

while True:
    time.sleep(max(0, next_delay - overhead))
    next_delay = odg.next_frame()

    display_bus.send(42, struct.pack(">hh", 0, odg.bitmap.width - 1))
    display_bus.send(43, struct.pack(">hh", 0, odg.bitmap.height - 1))
    display_bus.send(44, odg.bitmap)

That's it. It works. But of course not on the SAMD21, which has way too little flash and memory to fit it all. I have built a custom version of CircuitPython that has almost everything disabled to fit gifio, and it kinda works, except of course:

Auto-reload is on. Simply save files over USB to run them or enter REPL to disable.
code.py output:
Traceback (most recent call last):
  File "code.py", line 9, in 
MemoryError: memory allocation failed, allocating 24024 bytes

Code done running.

Press any key to enter the REPL. Use CTRL-D to reload.

I'm sure I could tweak the gifio library to make it use smaller buffers and make it fit in that RAM. But I don't want to.

I also don't want to write the whole thing in C, using tinyUSB and a gif library. It just feels too much like work, and very little like fun exploration.

Note that I will probably get back to the idea of a pendant with a display showing animated GIFs. Especially since it's now literally just a few lines of code in CircuitPython. But I will probably use a different displays and definitely a more powerful microcontroller. And I will possibly just use a ready development board, such as the Seeedstudio Xiao, because there is very little I would gain doing a bare chip thing from scratch at this point (and because I got spoiled by SAMD21's minimal component count, and any other microcontroller feels like too much work now).