If clock_t is used, normally results will show the database is "blazing fast" because it does not count the time spent waiting for the hard drive to actually save the data.
You may need to look into CLOCK_MONOTONIC. clock_gettime(CLOCK_MONOTONIC, ...) will measure "Wall Clock" time.
Any particular DML tests you’d recommend for the benchmark? Or thoughts on this in general.
Perhaps you can start with comparing 1,000 individual INSERTs (each with its own COMMIT) vs. 1,000 INSERTs wrapped in a single BEGIN/COMMIT block. This test is particularly interesting for comparing different RDBMS because it reveals how each engine balances data durability (ACID) against performance. You will likely find that some DB have a much higher tax on commits than others, which is a key factor in choosing a DB for a specific use case.
Since you are in C, please make sure to use prepared statements. If you send raw SQL strings, you are partially benchmarking the string parser of the DB rather than the actual data manipulation. Also, remember to "warm up" the DB with a few hundred runs before you start the timer, otherwise, the results will be skewed by the initial disk-to-memory cache loading.