How to fill this gap?

I am in the process of insulating this thin plasterboard wall, which is a few inches thick. To do this, I bought double plasterboard with 13 cm (5 inches) of polystyrene, which I will cut and glue. No issues there (photo 2).

The problem lies in the remaining space between the beam and the insulation (photo 1), a triangular gap of about 10 cm (4 inches)

I am hesitating between:

1/ Stuffing everything with low-density glass wool for attics. This doesn’t seem like a good solution at all because of compression and the likelihood of poorly filled spaces.

2/ Buying dense glass wool for partitions, cutting it into triangles, and pushing them in little by little. This seems to cause a lot of waste and requires precision, but it still seems better than option 1.

3/ Crawling behind the partition, making holes, and using expanding foam. I have never used it on such a large surface (i have 10 linear meters to manage - roughly 32 feet), so I don’t know if it is effective or even recommended.

What do you think? Any other option?