Youmu's important life goal is never stated explicitly in canon, but I'm sure it would be something along the lines of becoming a worthy servant for her mistress.
One of her main characteristics in canon is immaturity/inexperience and I think that her life goal is essentially the complete antithesis to that.
I wonder if she's ever meant to reach it.
When Youki disappeared, Youmu thought of it as a lesson from him. I think he will always be her role model in life and she will still need hundreds of years of training to even come close to his skill and experience as a guard.
But of course, being a worthy servant isn't just about swordsmanship.
She also shows interest in refining her other qualities as a servant.
She's specifically shown taking an interest in Sakuya's cooking.
>>69829
Even if Flandre really was that dangerous, I still don't understand why she can't leave at all. You'd think that she would be fine around other strong characters, especially the protagonists who had faced her before.