At this point, you may also consider creating a KickStart script that loads the basic configuration with mentioned API keys, groups etc. into a fresh FA installation. You've mentioned that you are working on a test installation, so during the further tests you will probably fill it again with new test data entities. If so, it will most likely be easier to recreate the FA with basic configuration via KickStart, rather than to pick unnecessary entities and delete them one by one. Such script also stands as a good documentation on what configurations has been made to meet your environment requirements.

This post may be a bit off the topic, but from my experience such approach quickly pays off, so you can at once solve your issue and boost up your testing environment ergonomics.

Unfortunately the default tenant contains the FusionAuth application and so it cannot be deleted, short of dropping and recreating the entire FusionAuth database.

If you use the request body you should be fine. You can do a lot more than 500 if you like.

I would start with 5-10k and see how performance is.