ᐅ Sliding door in the knee wall (under the roof slope) with drywall panels

Hello,
I have a sloped ceiling that goes down to 55cm (22 inches). I wanted to install cabinets there for storage.
Then I thought of simply attaching some brackets to the ceiling, installing a sliding door track, and hanging two doors there.
This way, I would have storage at the back and everything would look neat.

I have a 12.5mm (1/2 inch) thick drywall panel with a wooden substructure. The sloped ceiling is 260cm (102 inches) long, and there should be five rafters spaced 65cm (26 inches) apart.

Since I always find it hard to estimate loads, my question is how to best execute this without the ceiling collapsing.

My current idea is to screw a 300cm (118 inches) long multiplex plywood board, 15mm (5/8 inch) thick, which I have available, directly onto the drywall and into the five rafters. Then I would attach several brackets there to mount the sliding door track.

The two sliding doors will each weigh about 16kg (35 lbs), and I want the load to be supported by the track above so I don’t have to damage the floor with screws or adhesive.

I bought these sloped ceiling adapters from Bauhaus.
Unfortunately, I realized afterwards that they don’t have a load rating and are actually designed for a system with rollers on the floor.