It sounds unbelievable that you couldn't append some CSV or binary record to file via an open file descriptor more efficiently than inserting into SQLite.
(It's clear that to have the data already in SQLite form saves post-race steps.)
SQLite is the only solution for me so far that provides a structured data store for mixed data types, is power loss safe (important to an embedded system,) and is a high-quality and portable industry standard. Let along the SQL language itself, which has proved to be hugely useful for first-pass post processing given the amount of data that I ingest.
Perhaps there are other technology that will work just as well or better: maybe Protocol Buffers and alike? Yet SQLite currently ticks all my boxes and I'm quite happy with it.