If you are doing IO in a non-atomic manner (seems probable), it’s a matter of time before something happens and a file gets messed up. The frequency of that occurring is certainly a function of scale, but I would argue it’s always a bad idea to deal with your persistent state non-atomically.

Additionally, having a schema (even if it’s trivial) protects you from other software problems. It’s much easier to reason about upgrades. And more importantly, if your application complexity suddenly breaches the data complexity threshold where JSON files become a lot more difficult, you already have all the tools you could need at your disposal with SQLite.