I think you're onto something with the lighter washes or distressed finishes. A couple years back, I helped a friend redo her attic space into a cozy reading nook, and we went with a whitewashed finish on reclaimed beams. It really opened up the space visually and gave it a whimsical, almost cottage-like feel without feeling heavy or cramped. Plus, lighter finishes tend to bounce around whatever natural or artificial light you have, making the room feel brighter and more inviting.
Mixing beam thicknesses could definitely add some visual interest, but I get your hesitation about headroom. Maybe you could try a subtle variation—like slightly thicker beams in strategic spots (corners or around lighting fixtures) rather than throughout the whole ceiling? That way, you get some character without sacrificing too much height. Either way, sounds like you're already thinking creatively about it, so I'm sure it'll turn out great.
