curious if you got any further with this - just starting to research how to get things going with an Onion Omega Pro - inspired by @Tony_Smith’s tube-design i had seen on the TTN forum i’ve also gone looking for emmc-based options and ended up with the onion.
(gerne auch PM auf deutsch 
)
The Onion is an MT7688 so based on an SPI flash not an eMMC - in actuality that probably makes it a bit more robust than even an eMMC.
You can probably with some effort adapt the codebase for RAK’s MT7628 based kits. Beware there’s a kernel SPI patch needed, and then you need to make a corresponding SPI patch to the packet forwarder’s LoRa HAL.