Now that I have a few hundred km on this I thought I'd follow up with the drivetrain choices. Using the 38-26 rings vs. the 38-28 is completely seamless, it's butter smooth either way. Setting up the M8000(SG) RD with the 11-34 cassette wasn't too bad. IIRC Shimano specs 6mm of gap between the largest cog and the guide pulley. IME this makes it a little harder for the chain to ride up onto the largest cog and a setting closer to 8mm works better. Setting up for this cassette with the B screw completely backed off I get 9-10mm of gap. Shifting is just fine. As an unexpected bonus the guide pulley tracks the smaller cogs better given the shallower slope on the cassete so nothing lost here either.
One thing I would mention if anyone else does this is I did the usual stretch the chain large to large then add 2 links to set the chain length. Just so happened this exactly lined up. Going large to large (not that I'd ever do this in regular riding) the RD is strangely stretched out, nearly parallel with the chain stay. Again it works fine just looks weird so I'd probably go for +3 or 4 links.