>> Digitization creates a strong barrier to any trickery.
Again, please explain how exactly. No one is actually proposing reading the letters anywhere, if for no reason other than the insane amount of labour that this would involve. Even if they did, inmates who want to conduct nefarious activities will use code and send perfectly innocent messages that no prison worker nor AI will spot as that. So you're going to spend millions of taxpayers money reading people's letters with literally nothing to show for it.
So your options are:
1) Spend millions scanning and reading every letter, which won't actually do anything to prevent criminal activity in and out of prison
2) acknowledge that letters maybe should remain private and save yourself millions analysing useless noise
3) go full on totalitarian and forbid prisoners communicating with the outside world completely(and ignore the fact that they will anyway, because corruption within prison system is a thing), dealing immeasurable harm to people in the prison system and their families
And finally - if you think about it for a second, all the arguments you used can be used against in-person visitation. Should the in-person visits be monitored and recorded too? After all, what if the prisoners issue commands for people outside to commit crimes in those meetings, we have to record everything just in case, right?
Again, I'll repeat what I said - all of the above is just a scheme to stuff private pockets with taxpayers money, at the cost of misery inflicted on prisoners(because hey, they are prisoners, they don't deserve any sensible treatment, right?)