x470 Taichi memory issues

remove all ram sticks
gently clean the brass finger contacts with a pencil eraser and then ideally isoprop alchohol
with no power at all to baord and bios battery removed spray isoprop in the ram slots and leave to totally dry out

you issue usually is 95% bad contacts from ram stick to slot
it could also be be much less likely you need to reseat your cpu (bad contact on some of the pins used by the ram)

the 1866 speed issue is strange for a few reasons
1) this is quite a rare 'jedec' default speed indicating very very early ddr4
2) a 2700x and tachi should make a good effort to run 3200 (even with 4 sticks,it should try all be it maybe unstable with the 4 sticks)

my advice cure the issue with 4 sticks not being fully used by windows in the first advice i gave you

then try to set the ram at 2666mhz >3000mhz >then 3200

allthough you will for sure need the latest bios you can use with a 2700x to have much hope of 3200mhz with four sticks of ram

EDIT
i have just noticed in the last screenshot the ram is actually jedec 2133mhz.......this is very strange to default to 1866 (meaning i cant understand why it even has instead of just not booting full stop)