I do something similar for an audit trail at work. I work with the type of data where we may need to know who looked at what and when. All those records are stored in a separate SQLite DB (main DB is postgres), and I cycle it out once per calendar year. That makes archival trivial, and should a compliance person need to look at it a simple desktop app can open the file easily.

You can't beat SQLite for ease of use. I'd try it out and simulate some load to see if SQLite can keep up, if you keep your inserts simple I bet it can.