Thanks for the link, I’m aware of reproducible-builds.org.
Both your causes seem trivially fixable here - the QEMU builds could have a standard system clock time they start with, and an ‘unsorted’ file listing made in a deterministic OS environment will keep the same file order, no?
By comparison the rb.org site says you need to start with stripping all that stuff out of your build process, for the reasons you refer to.