[Delphinus]

Thanks guys. I haven't ruled anything out at this point, seems like there are plenty of options.

The amount of flex shows up the most on the corner of the bulkhead. The door wall doesn't really extend upwards into the joist space as much as it looks like it does in the photos. Well, it does extend up into there, but only in that it the top piece is resting on the exterior wall (same shelf that the joists sit on). I have 2x4's flanking this piece holding firm to the adjacent joists. There is some flex but it seems like only about 0.5cm? I wasn't worried about it until you mentioned it.

(Having said that, I'm not sure I'm still terribly worried because I was hoping to go with a drop ceiling instead of drywall ceiling.) Nevertheless I can put some more studs in place as you describe. Unfortunately I don't see this solving the bulkhead bounce issue, but I think I'll try the joist-sistering and see how it goes. I was thinking maybe sistering the one joist in question that seems to be giving the most bounce, and then cross bracing after that right at the spot. (Disclaimer: I'm still reading up on that method so if I say anything here that doesn't make sense, it's because I haven't finished my homework on that one.)