x370 Taichi doesnt' boot

You have the memory incorrectly installed try A1+B1 and if that doesn't work A2+B2. Other steps you can try;

1. That HD3850 is a very old GPU do you have something more modern you can put in to test?
2. Ensure all connections, devices, and the memory is seated correctly.
3. Unplug the power cord, remove the CMOS battery, push power button 3-4 times to discharge the capacitors and wait 30 seconds then try to boot.
4. Try one stick of memory in each DIMM bank to see if the system will boot, repeat the process for the second memory stick.

^Also this, not sure how I forgot to mention that

Unfortunately yes. I am not sure AMD still gives out boot kits for X370 though.
If memory serves they gave out an A8 9600 or A6 9500 with the boot kit for X370.
Looking at the X370 Taichi CPU support list these CPUs are not listed as supported.
I believe they do work when used with a discrete GPU but were left off the list
because the X370 Taichi lacks display outputs. If you can find one very cheaply
it may be worth trying, otherwise, I would stick with the r3 1200 to be safe.

when you get the r1200
my advise is
one ram stick in slot a2
and fully reset the bios to default (eg remove the coin battery from the x370 for 30 mins)
boot to bios and find out EXACTLY what bios version you have before doing anything
depending on version of bios you may have to flash to a bridge bios circa version3xx AND THEN AGAIN a bridge bios version circa 5.xxx

DO NOT TRY AND SKIP THESE BRIDGE BIOS VERSIONS
they have important changes to make before flashing later bios versions

keep just one stick of ram in slot a2 while flashing bios versions it helps prevent issues updating bios and then the initial booting after bios flashing

Good catch Datonyb, I missed the RAM being ECC. If you still can't get the system
to post with the 1200 then you will need to try some standard RAM.