I would say stick with Astera since it already +15 already, maybe work on synth to get max stats
But since you have Mill gear already, get the bottom, a lot cheaper to reforge due to smooth being cheaper than solid and stick with 3 astera / 3 mill combo.
Main reason in my opinion is that ...MIll is still stupid crazy to synth for. It only takes New Era gear, and you barely get any in Red moon and you try to farm Heavenly mats but drop rates are so low you can spend hours just to level one point. Oherwise some new enchants would be nice. Overall just getting over the new wall of Quick Battle requirement then work on Crit , Balance and attack limit release