You were on the right path by going with the Technic harness and JL XD600/6 amp, but going with the OEM EPS amp would be a step backwards IMO since that amp is designed to work with the factory EPS headunit which sends an eq'd signal to it. Factory signal from the HiFi headunit to the factory HiFi amp is a clean (un-eq'd), 5v signal you'll get once you bypass the factory amp with the Technic harness.
Curious, why not spend roughly the same if not a little more on a aftermarket 4" component set for just the front stage and leave the rear and under seat speakers as is? You'll notice a significant improvement in sound from the OEM HiFi 676 system with the JL amp upgrade along with the front stage upgrade and powering the OEM rear and underseat subs with the remaining 4 channels of the amp while keeping cost down.
For the money these would be hard to beat, and direct bolt in:
https://www.crutchfield.com/p_975MS4...42C-BMW-1.html