I too went through the trouble of “BOOTMGR not found. Press Ctrl+Alt+Del to restart”
Then I imploded and realized that you need to use the Data Migration Tool from Samsung.
This will clone your old HDD to the 970 EVO in one shot, making it bootable right after.
Forget the boot installation with the USB Stick.
Before all that, make sure you activated the SSD in the Partition Management window and assigned a volume and a letter like D: to it.