Afaik there is no way to collapse the ToC in IREX pdf viewer.
I'm (slowly) working on an alternative experimental pdf viewer [1] that it does have that feature.
Note there are tons of missing features (zoom, annotations...) and it's slower that default pdf reader.
Maybe some day I could finish it.
Iņigo
[1]
https://www.mobileread.com/forums/sho...d.php?t=154409