After you delete an account, you have to use your primary account to re-enable it and then use your secondary email to for that account.

We will respond later on please keep updated.

I didn't delete my account. I simply wanted to change my FF account that I made at work to use my work email. So that FF account now has one primary email, my work email and no secondary email listed. Because I deleted this email (personal) from my account.

But If I try to use my personal email, which is now not associated to any FF account at all. It tells me I can't create one and tries to sign me into FF account and says "Primary account email required".

This email shouldn't be associated to any FF account. Because I deleted it out of my FF account from both the Primary and Secondary email.

Try to delete that account to see whether that frees all involved email addresses and then create two accounts to be used for home and work.

I'm not sure why you want to use Sync if you only have one device connected to a Firefox Sync Account or do you have other (mobile) devices that you want to connect to either Firefox Account?

Keep in mind that Sync is optional and isn't meant to be used as a backup, but it meant to sync data between connected devices.