Out of all that you listed the Seekins iRMT-3 Upper and Ambi lower are the best option in all categories, Price, Quality, and function. I used JP Micro Fit Takedown Pins and hand fit them creating a rock solid pairing.
The iRMT-3 upper is excellent for a precision rifle setup.
I'm just waiting on the Bartlein barrel from CLE, this rifle will be identical to the GA-15 and $1,150 less than what the GA-15 sticker price. This is a prime example of how much you can save by building your own.
I am making a few slight changes, I'll be running a superlative arms AGB instead of SLR, TriggerTech New Single stage trigger and a APA Gen3 MB which you would have to pay extra on top of the GA-15 $3150 price tag. Bringing this entire build to just barely over 2K.