I started to reply. But then I thought I would re-read the question. Which is;
"Are there any ideas that would work similar to a database other than a simple text file?"
and to that, my answer is no.
I have ideas how I would handle database transactions, with store and forward guaranteed message concepts, involving simple text files, but you asked for something else. That gave me the impression you already know how to do it with simple text files, and I really can not add.
For other readers, who may not know how to use simple text files; identify how you can make a db transaction atomic - ie self contained.
Look into things like diff files for ideas and mold the idea to fit your data - I have no real idea what your data looks like, so I can only suggest starting points.
Once it is atomic, write it to a separate disk file on the SD card.
A different thread, reads ALL files on the SD card one at a time and attempt the transaction with server/database/both. When confirmation is received that the database transaction successfully completed, the file should be erased from the SD card. This method will survive system and network failures. If you need to prevent accidental double updates, include a sequence number. If it has already been used indicate a previous transaction succeeded.
