In this case, this probably won’t give you much more information, but erlinit can provide a shutdown report that’s saved to /data. Add this to your config.exs:

config :nerves, :erlinit, shutdown_report: "/data/last_shutdown.txt"

If you want to get a crash dump from Erlang, then environment variables can be used to control that. You might already have this or something like it in your Nerves system’s rootfs_overlay/etc/erlinit.config, so check that first:

# Enable crash dumps (set ERL_CRASH_DUMP_SECONDS=0 to disable)
-e ERL_CRASH_DUMP=/data/erl_crash.dump;ERL_CRASH_DUMP_SECONDS=5

I feel like the sad part is that this is going to end up being an issue with a NIF. Take a look at this blog post for working with core dumps with Nerves.

The only other idea I have is that if you have a suspicion on a possibly sketchy NIF, perhaps there’s a way of calling it a lot that can make the crash happen more quickly.

A shot in the dark, but for what its worth …
Feels like it may be due to some resource (storage/IO buffers etc.) filling up over time ?
Have you checked log files or terminal output (stdio/stderr) from daemons that aren’t directed to /dev/null ?