I patched this mod on my end to allow myself to use the machine allo portrait after synthetically ascending (I did it by changing the portrait in the create template bit in the species menu)

in zz_saurian_portrait_categories.txt you add this into the document:

synthetics = {
name = SYNTH

sets = {
syntheticsaurian
}
}

then in zz_saurian_portrait_sets.txt you add these lines:

robots = {
species_class = ROBOT

portraits = {
"machine_allo"
}
}

syntheticsaurian = {
species_class = SYNTH

portraits = {
"machine_allo"
}

randomizable = no
playable = yes
}

It should let everyone use the robot model properly if you replicate these changes
Cheers!