A Garlemald expansion would've dragged on even worse than early Shadowbringers or all of Stormblood did. They simply (messily, mind) resolved too many plot threads pertaining to it in ShB. There was very little left and it's only in retrospect of EW that people can tell themselves otherwise.

If there was a time to do a Garlemald expansion, it was 5.0. Not 6.0.