How do I remove malware from all my Apple devices, including iPhone after installing a developer app?

Sorry, but you have not provided any information that would support such a claim. There is absolutely no way for anything to be installed on the System Files of iOS as it is locked on a read only partition of your drive, so even you cannot make any changes. RAT's are just a buzzword used to scare people and I suspect you found something in your logs where it actually refers to Radio Access Technology and nothing to do with what you think it is. Infecting Bluetooth devices is also a common fallacy that is seen on Social Media sites when people end up going down a rabbit hole.

If you are worried at all, create a brand new Apple Account and reset your device to Factory Settings and use that account. You will lose all data, but it is a fresh start and extreme approach that is probably not necessary.

Stop using the phone. Turn it off. Take your full evidence to the Police.

If you believe your personal safety is at risk, or your iPhone is compromised contact your local law enforcement and the FBI. Hacking is a cybercrime and investigated by the FBI.

Apple will review requests for cooperation from when submitted by law enforcement.

There is nothing that we can do to assist other than offering moral support. I hope everything works out for you.

https://www.apple.com/legal/privacy/law-enforcement-guidelines-us.pdf

https://www.apple.com/legal/privacy/gle-inforequest.pdf

Apple is not an investigative body, so your best bet is to contact the Police, FBI, or hire a security specialist for any criminal matter.