Hacker's persistent access to iPhone despite burner phone and replacement

With everything that has been provided it would appear your accounts have been compromised, not the phones. For your Apple ID, you must have 2FA and make sure the Trusted Phone number on that account is one of yours and not someone else's. You can opt for a higher level of security using a Recovery Key. This will give you a 28 digit alphanumeric number you will need to write down and do not lost otherwise you will not be able to regain access to your account if the password has been forgotten. No one else will be able to reset the Password without the Recovery Key.

Set up a recovery key for your Apple Account - Apple Support

For you email accounts, you will need to definitely change the passwords for them, but inquire about additional protections they can provide to prevent access from others. They should be just as concerned about the intrusion into their network as you are.

Based on what you’re reporting, you’re worth millions of dollars and some exceedingly advanced tooling to exploit.

This puts this entire discussion far outside of what can be addressed by Apple, by the folks here in this forum, or by anyone short of those with forensics-level access.

As you have been told by multiple sources.

You can certainly persist in asking of course, but you’ll find the same suggestions arising from each request, and you will have undoubtedly already considered and acted appropriately upon those suggestions.

You’ll either need to pay for those forensics to be performed (and then also finding some indications of compromise), or learn to collect forensic evidence yourself (and finding evidence), or can continue with the current situation, or you can migrate to a feature phone. That feature phone migration certainly won’t prevent security issues, but it’ll reduce what information is available on the phone.