x370 Taichi doesnt' boot

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.

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

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.