> I'd set up attractive repair shops around army bases and government offices (quick turn around, competent service, reasonable prices) and just mirror every phone that comes through. Most of it will be useless, but I'm sure that one can occasionally find some gold (in the form of documents or compromising material).

1) I would be surprised if that isn't already happening.

2) NOTHING is useless in intelligence work like that. It doesn't have to be directly and immediately useful, but you can learn an enormous amount through so called patterns or life analysis.

3) Information leakage via devices like this is an enormous problem. C.f., leaking the location of military bases through Strava logging or nuclear weapons storage information via flash card study apps