I think it's going to be very rare that a database changes enough for a plan to be significantly worse than the optimal plan during the life of a prepared query

I think it's going to be very rare that a database changes enough for a plan to be significantly worse than the optimal plan during the life of a prepared query. A database engine could easily mitigate against it by re-planning queries every 1000 runs of the query or have the query plan expire after 60 seconds or something. It might be worth trying to re-prepare the query from time to time as part of this kind of bulk insertion benchmarking. The other thing that would be good to try is closing the transaction every 1000 inserts or some other number. Doing several inserts in a single transaction is certainly better than 1 insert per transaction, but there may come a point where the transaction being too big starts to slow things down.