Sierra install on older Mac

I recently replaced the 1000 GB HDD on my 21.5” iMac (late 2012) with fusion drive (sn: D25L50RYDNMM) with a 480 GB SSD. I was able to separate the new SSD from the fusion drive 128 GB blade drive so that the new SSD would be the boot and data device. I am trying to install OS 10.12 Sierra, the last OS to support 32 bit apps.

When I boot from a USB with OS 10.12 dmg image and I get to select Language etc, I get a screen saying “Something went wrong…” or on occasion, it locks up.

I tried to boot from an image of 10.11 El Capitan with the same result. It will boot and install from an image of 10.13 High Sierra, but 10.13 does not support 32 bit apps. 

If I boot using Shift-Opt-Cmd-R, it installs OS 10.8 Mtn Lion, the OS it shipped with. Hoping I could upgrade from here, but it can’t open the App Store, Safari is quirky, only opens Apple Store site. Other attempts say “Can’t open this page”.

If I boot using Opt-Cmd-R, It installs OS 10.15 Catalina and runs fine, but again only supports 64 bit apps and won’t run my 32 bit apps.

I reset the NVRAM and tried the above again, but no joy.

For some reason it’s allowing me to install only 64 bit OS systems.  No help after lengthy internet searches.  I wonder if there is a setting that needs to be added/deleted/modified via Terminal.  I know this is fixable, but my aging brain is tapped out.  Surely there is an Apple genius out there that knows the answer.  PLEASE hear my cry for help. I contacted Apple support by phone on 2 occasions (Case ID: 102230311607), but got no solution.