A database optimization I made was changing a tables id generation from a manual generation scheme (some other table had an entry with the next usable id, it was updated with every entry written) to a uuid generation scheme. The table stores data from a daily import, on a fresh import all previous data is deleted. On some systems, there are more than 10 000 000 entries to be imported on a daily basis, which took 8 hours. Now, with batched inserts and the mentioned improvement in the db scheme, it’s at about 20 minutes.
TLDR: Reducing the amount of queries sent is good, because although network is usually fast (ms), db requests are still slow compared to the speed of an application (clock cycles).
And yeah, there is an option to only import changes daily, but sadly that isn’t supported in every environment.
You might try advent of code. It’s not a project though, but good for improving your skills imo