You seem to be turning around the DDR behavior.
Make a simple test: go to the bios, select the slowest non XMP speed available in the DDR profile and test if all the system is booting well.
If it is booting well and stable: run the DDR at low speed until you by a new set more performant.
If it is not booting properly, it is that the issue is elsewhere.
For your tests with 2 different motherboards, it does not seem that the X370 Taichi is faulty.