Hey there. I love to see a project like yours!
I know you are on an early stage of the project and you want first give support for the 5th gen Kindel no-touch. But I’ve got a PRS-T1 from 2011 / 2012. The newest update from Sony is from 2014 (1.0.07.05070 - released on 19 June 2014). Is seems to be the latest one and I would love it to see this device is not landing in the garbage.
Sony had some other devices with the same Freescale mx chip:
https://de.wikipedia.org/wiki/Sony_Reader#Technische_Daten
So the PRS-T1 , T2 and T3 seems to have the same chip. A i.MX508. Here is a info paper:
https://www.nxp.com/docs/en/fact-sheet/IMX508FS.pdf
The guys from mobileread do a lot with this device. I was able to root it but with the Android froyo on it. :/
https://wiki.mobileread.com/wiki/PRST1_Rooting_and_Tweaks