> all you need is a trusted image
I bet you, that image will be provided by the trustworthy people from NSO, free of charge or at a price! Whatever makes you trust their image.
IMHO devices should be root-able but with high barriers of entry, something like soldering should be involved. If you are after doing something that you don't understand but a stranger on the internet told you to do it you shouldn't be able to do it.
I just want to remind you that quite recently a few police agencies come together, built a "secure messaging app", fed it to the criminals and tracked all their communication until gather enough information to take down their entire operation.[0]
Or the time when CIA run a Swiss encryption company[1]