What a delicious challenge! We're with you, Pete!
I lived in an attic room with slanted ceilings for years, so I know you are crowded!
The first place I would begin would be those things you say are "going". Your car speaker, the old TV, skateboard, old car parts, the penguin. Also: Get rid of your old university papers. OUT, today!
Then, you really need to get that large pit behind your Ikea "closet" opened up. That is great space! I can't see how the Ikea units are convenient for storage at all. No wonder you don't want to put stuff away. Take all the clothes out and purge what you don't wear.
Although I typed most of this before I noticed your floor plan under your video, I am making the suggestions as optional ideas. I would hang a closet rod on the tallest side of the pit (Left wall behind the staircase side) for your clothes. The ceiling is not tall enough to do a double rod above and below, so either hang the rod low and add shelves above, or hang the rod high and put your Ikea storage units below. If you hate looking at it, hang a curtain across the whole area.
This leaves the right hand side of that pit bare. Place a strip of pegs along the knee wall and use them to hang your photo equipment, tripods, cables, spare things from the opposite corner you got on e-bay (sorry, couldn't make out the item name)
Purge those DVDs, then put in some storage that fits the size of the DVD boxes that remain. I have a tiny closet that holds hundreds of CD or DVDs because of the shallow shelving I bought <http://www.technooutlet.com/at081.html?mr:referralID=cf3f5cc6-3e0a-11e0-a86a-001b2166c2c0>
I believe these would fit against the wall on the other side of your stairwell.
Get rid of the table on the corner to the left of your desk. Put the bookshelf there and use it to house the boxes you have stored under your bed. Remove the stuff from under the bed and the things behind the knee wall next to your bed and dump them. Looks like these things are not critical to your current lifestyle.
The drawer unit under your desk is in a place you can't access. Place it in the dormer cove and use it for the electronics such as the old XBox and cables.
Let us know how you're doing.