We’re in a mixed marriage, too - I don’t like clutter, and Jim likes all of his stuff kept all the time. Your past definitely plays into this - I moved with just my car a couple of times as a young adult, and that got rid of a lot of childhood clutter and got me comfortable with the idea of “Out it goes, and if I need it in the future, I’ll buy a new one.” Jim had a traumatic experience as a child with his parents throwing out his toys in a cross-country move without consulting him, and a couple of moves back and forth across country that upset his equilibrium, and now he likes to have his things around him. For me to throw out his stuff behind his back really would be a betrayal.
He’s not hardcore, though - he’s open to de-cluttering when we pack up and move house. The idea of me going through his desk and asking him what to keep and what to throw might work - de-cluttering is work, plain and simple, and no one really looks forward to doing that job alone. I also like the idea of making piles of his clutter for him, and let him be re-active rather than pro-active - let some light in on the old, musty piles and see how they look to him then. We also have our own clutter zones - I have a room, and he has a room, and we decorate and clutter up those rooms to our heart’s content (within limits - no bugs, fire or health hazards).