Which already exists and has existed for ages.

Lineage os (Android), e/os (Android), postmarket os (straight up Linux) would all be excellent answers for cutdown/debloatable mobile phone OSes. (also Ubuntu touch and whatever the pinephone shipped with)

Its probably a lot harder making an unintrusive os than it is making this.

This is cutdown hardware leading to simplified and cutdown software as it's a necessity here.