As a person who started with the Old 'Regalia One' (The variant below Regalia) back in 2020, I would suggest to take the Regalia Gold, as it is better than Millenia in every aspect (unless you are into cashback game and not into the travel/hotels/miles game).
I upgraded from Regalia One -> Regalia -> DCB -> Infinia.
If your end goal is Infinia, then Regalia Gold is your best bet any day compared to Millenia.