I think your option 1 is partly the way to go, take some of it out, shim underneath and reinstall. More info on this and how it's done in the following links.
From our glorious DIY neighbour: How do I use 'surface shims' to level out a sloping floor?
flattening subfloor with plywood! on the Floor Pro forums.
How to Lay Subfloor on an Uneven Floor on SF Gate.
If I do #2, will a random orbital sander take forever to remove 1.5mm of hard plywood from about a 40"x30" plane? I know a belt sander would be better, but I don't own a power sander right now, and would rather buy a random orbital for future jobs.
If you did want to go ahead and remove material an ROS fitted with a coarse disk (60 grit or even lower) would do it but obviously a lot slower than a belt sander.
Possibly the ideal tool to take off that much material from plywood in situ is a hand plane, even a small block plane would do it if you were patient and careful. Because of the layered construction in ply and the nature of most modern veneers the work isn't anything like as laborious as it would be removing that much material from solid wood.