I am about to commit sacrilege here, but I just want to point out there’s no law that says you have to level the best available hero. If Domitia makes you happy then no-one can stop you going for her. It’s also possible for a “lesser” hero to have better synergy with your existing team than the newest shiny one.
I’m not trying to persuade you not to level the ninja, mind you. A way out of the painful dilemma is the classic getting them both to 3.70 and playing around with them for a while. You might (to be fair, you probably will) find Onyx is just too good/too much fun to pass up, in which case promise Domitia faithfully that she can have the following set of tabards. I’m sure she’ll forgive you.
Baldur is tough enough without emblems IMO. I don’t have him, but he’s annoying as something you’re not allowed to say on this forum when I meet one. Maybe throw him a line or two just to give the special a chance to kick in?