Another DIY repair, non-barrier tubing

Hi all, I was wondering if anyone has an install photo to share where non-barrier tubing was used and a heat exchanger was necessary to split the system. I've done it a thousand times with atmospheric OWB systems, but never with a wall hung unit and am trying to picture a better layout than my little brain can come up with.

Customer installed 2 loops of Dura-Pex in his garage slab and another 2 in his kitchen, 60 feet away. 50 gallon electric WH in the garage with an 007 literally dangling from 1/2" Dura-Pex off the relief valve port with both domestic ports on top capped. No expansion tank, air eliminator, or any means of control besides one of those inline 120v stats you'd use on a Torpedo heater in the garage to run the circ, so the kitchen only gets heat when the garage gets heat. Oh, and both 300ft loops in the kitchen are fed across the basement with 60ft of 1/2" from the garage. I was called out to discuss adding the new 1300 sq ft detached shed to the system and integrate all 3 spaces to a single new boiler (detached via foam filled underground from the basement, 120ft) and possibly add 800 sq ft of underfloor under the living room. We've since settled on a separate unit for the detached to avoid the underground piping, but the non-barrier tubing in the house and garage still pose an issue and we've settled on an Electro boiler which is mild steel. My drawing and calculations say to separate the boiler from the emitters with a 30 plate HX and basically pipe it P/S with the HX being in place of the hydro sep, with separate expansion tanks and reliefs on each side of the plate. What say you?