I have a personal profile for a business as well as a business page. I plan to delete the former, but unfortunately that's the one that has the desired username. If I delete that profile, will the username become available, or will it be lost forever?
Yes even your Username will be free for example
After deleting your account any one can use that username, so you'll lose it if someone else registers it. If you want to retain it, deactivate your account rather than deleting it.