Once you move the SSD to the new motherboard, you'll lose the license/activation if you don't have it tied to your MS account.
Also, your new board may be different enough to cause startup issues. Even if the system starts up when connected to the new board, it will need the new drivers installed and possibly old files and the registry cleaned up. That is why a clean install when moving to a new board is preferred.
Cloning the 3.5" SATA drive you apparently have now to a new M.2 would be easy enough, but you can only use an M.2 (SATA or PCIe) if there is an appropriate NVMe slot for it on the board. The Gigabyte X270X-Gaming 9 has that.
Yes, generally M.2 PCIe is faster than SATA. There are exceptions; some PCIe are just cheap and slow.