Wow! Well you never know where simple frustrations will lead, or in your case noticing something that you just can't shake that no one else seems to think is important. I'd say keep me posted, but that's not on you especially while you're developing that app. I wish you the best of luck, and it sounds like you're doing it with a really unique and authentic perspective that I wouldn't be sure that any of the apps that become popular on either App Store can guarantee. Seriously, the world might depend on you :)

I had a feeling about what you described with GrapheneOS would be the case, and that's what kept me from really considering it as a replacement for my iPhone until talking with some folks in this thread. I really don't see myself getting out of using an iPhone as my "main phone" tied to my phone number since my wife is neck-deep in the whole Apple ecosystem (and I truly believe that being flexible in this regard is worth it and makes our lives a whole lot better, even when the issue in question is what I would consider a simple moral non-negotiable, securely protecting my and my family's personal data. just means that I have more solving to do before the solution).

My solution for now is to always run everything through a trusted VPN and NextDNS on the iPhone, or as much as iOS will let me I guess, and using this as my new Pixel's gateway to the internet when I'm away from a trusted connection. I will also be running everything through the VPN when I'm using GrapheneOS, so when I am out and about I'm not treating my not-entirely-trustworthy iPhone any differently than a Starbucks hotspot. Sometimes the convenience really makes a difference, not all the time but it does matter occasionally.