I feel like this sounds worse than it is because you also don't create a Threads account, per se, in the first place. You use your Instagram account. If you want to use Threads, you activate your Threads profile; if you want to stop using Threads, you deactivate your Threads profile. Your Instagram account is still your Instagram account and exists independent of Threads. If you want a totally separate account for Threads, you need to create a second Instagram account for it.
PSA: You Can't Delete Your Threads Account Without Also Deleting Instagram (www.makeuseof.com)
Meta's Threads app has a major flaw: you can't delete your Threads account without deleting your Instagram account.