IMHO new character > constellations every time, but it would depend how much you want Eula. If you don’t want her, saving for Fontaine is a smart move at this point in time.

You should prioritize whatever leads to you having the most fun. Are you planning to use Eula on a team? If not, then it’d be probably be a better idea to invest in your main.

I personally value more characters, because you get a lot more variety and fun from having multiple characters then a slightly more powerful one imo. Also, not like this game is hard, and c0 Kazuha will easily clear all future content lol.