Why am I getting "An internal error occurred while preflighting your volume for APFS conversion"?
Try to install on the old disk first, to see whether there is a hardware issue with your new disk, for instance by copying a lot of files to it. You can also then install a second copy of your OS to the SSD and see where it fails.

Once both work, if you boot with ALT and press CTRL, it will save the ssd you are booting to as default.