Incrementally shutting off communications paths and disabling Apple iCloud access and other steps likely also taken here seem an expensive and problematic way to create yourself an ad-hoc feature phone.
Having already sought to lock down your iPhone into something approaching an ad-hoc feature phone, you can and probably will still have concerns over the security and integrity of your ad-hoc feature phone, and those concerns unanswerable. This because proving a negative — that an immensely well-funded adversary has not compromised this ad-hoc iPhone, or a purpose-built feature phone, or any other local devices — is impossible.
So… why bother paying for an iPhone that is then rendered mostly non-functional, all while still exposed to your perceived immensely well-funded adversary?
If you are correct here and you are a target, then you need tailored help with your security requirements and practices. Help well beyond what can be offered around here too, given the sorts of sensitive and private discussions involved.
And the sorts of exploits you are concerned about would be (are) immensely expensive, and targeted, and well past what the folks around here can assist with.
Within this thread so far, I see nothing indicating a security compromise.