There is SailfishOS[1]. It's real Linux. It has proprietary parts (e.g. Android drivers, UI widgets, core apps), but Jolla (the developer SailfishOS developer) has contributed a tonne to Linux on the mobile[2].

I use it as my daily phone, and I've developed apps for it, so feel free to ask me any questions.

SailfishOS should hopefully be available for the Sony Xperia X soon (within a month according to their blog posts).

[1] https://sailfishos.org/ [2] https://git.merproject.org/groups/mer-core/activity