You can download MIUI here: https://xiaomifirmwareupdater.com/archive/miui/hydrogen/
As for the Linux... I can't help you with it.
Phone feels sluggish compared to last month update. Specially when switching between apps. It can be a memory management issue or just my imagination
Also sometimes apps that I have chosen not to be optimized, get closed! For example Viper4androidFX and AccuBattery
Pixel Experience Plus has some additional features comparing it to Pixel Experience. I built it long time ago. As far as I remember, I added Face Unlock there.
I'm sorry to tell, but it couldn't be added like patch or anything else. I don't know this way if it exists.I love & cherish your work, as i am currently using/running it without any flaws but just few things which i noticed by using both versions of pe & one of them is face unlock which i surely know is there in pe but not in pe plus. So, i got anxious to know that can we add it after flashing the rom like some patch..