As Elaine said, this may be simpler in ChimeraX.  Nonetheless, if you have a fasta file (or equivalent) with your sequence in it, all members of the ensemble should automatically associate with the sequence when you open it.  Nonetheless, the remaining steps (showing RMSD header, saving attributes file) do not have command equivalents and would require resorting to some moderately difficult Python code.  I don’t know if you’d prefer to do that or switch this to ChimeraX where it’s more directly supported.  I can provide guidance on the former if you really want to stick with Chimera for some reason.