Most likely it is graphics drivers.
I gather from the stream of consciousness video:
- replacement mainboard w/same model - possibly same BIOS - possibly same version
- same cpu
- same graphics
- some overclock
- same OS boot media which worked with old mobo
Issues:
- unable to activate Win10 64
- code 43 graphics crash
If it were mine, I would:
1. turn off all overclocking
2. install new storage - but set BIOS to boot from installation media
3. install clean Windows 10 - partition and SLOW full format
4. confirm all Win10 updates installed - check updates until none left - test and activate (even if you have to call, do it now)
5. download and install proper graphics drivers (use DDU to remove old on any reinstall)
6. install steam and games on new storage
7. set old storage as secondary (slave) storage and copy game saves as desired (steam has directions for this)
Others might suggest just trying DDU and a fresh download and install of graphics drivers. Might work.
Your choice.