Sorry but I dont have the time or knowledge to do this and It would require the phone in hand to even consider an attempt at it.
I also have some very capable devices hardware wise but sadly no kernel source code for them so those are all stuck on older android versions too.
To be honest I find a stripped down oem stock Android 6.0 ROM better.
I swap all the oem's app's such as launcher dialer messages contacts etc etc and replace them with google's own app's and associated libs after this I find most rom's work really well and I get app updates on regular basis.
A softly softly approach is best when removing the oem stuff as you don't want to lose any real functionality so this part can be time consuming...