I’m in a similar situation as yourself on the audio upgrade. I’ve got the same non-Bose system (2020 Custom) as well.
I added front component sets and coax speakers in the rear doors (all Infinity Reference speakers). Stock HU feeding an LC7i into a Kicker Key 200.4 amp. The amp has DSP and time alignment built in.
Sounds mediocre at best. Really muffled actually. For the harness, I found one on crutchfield and made my own loop back. Very simple actually. Fed front channels into the LC7i and left the rear door channels alone to feed the rear doors of the stock HU power (not amp’d). The idea was to run the components in a bi-amped config (4 channels feeding front door speakers and new dash tweets all on separate channels) to take advantage of the amps time alignment capabilities.
Issue is the amplification of chimes and signal sounds. These come in from the front channel HU output. In my config I had to turn the gain at the LC2i down to limit amping those sounds. Problem is it’s limiting the regular audio signal at the same time. I’ve ran it this way for a while since it’s a PITA to remove the rear seat and I needed my truck back (ran out of time to F with it). Hoping someone with more experience than myself or with a similar config has solved this issue can weigh in and help yourself and me. I feel my issue is a tuning problem or an LC2i config issue.
Best of luck. Following your build.