Installing new sandisk ssd - created bootable drive but holding option during startup doesn't work...?
i would but a known good drive back in the Mac to ensure its working. Then install an is on the sandisk drive while it's connected externally via USB or whatever other dock you prefer. Once you're sure all the parts work, then move the sandisk drive internally. 

Much easier to troubleshoot hardware when you only change one item each step along the way and then test.