I kind of like the barbarian option better narratively from a pacing standpoint, but I do agree with that the underground is more interesting tactically (and maybe even narratively? ).
Let me explain why I like the barbarian option:
1) In many ways, Ling Qi's heroic efforts against the first major barbarian incursion was a demonstration of both her personal power as well as her inexperience. She chased barbarians away but never quote managed to score a convincing win against them. Like sure, she killed a bunch, but the results were mostly inconclusive.
2) It put a spotlight on all the glaring deficiencies in her own personal philosophies: from her views on family to the meaning of heroism and self-sacrifice and even the role she plays in times of war.
Narratively, then, it'd be satisfying for her to come back and demonstrate just how much progress she has made. Her field effects are far more potent, she's wargamed with Zhengui and can coordinate better with him and her personal defenses have been jacked through the roof. CRX and her working in tandem would call back to that initial bandit raid too.
It's a reasonable place for the narrative to go.
On the other hand...
There are no good memories of the underground. CRX got puppeted by her dress, Ling Qi is literally part dream, Zhengui lost one of his dreams, and fourth realm enemies hit others with forced pain gestalts...
The likelihood of things going pear-shaped and for another Rita Repulsa 'Make My Monster Grow' moment to happen seems fairly high, especially since the Shishigui have a combat doctrine that makes them go in hard. Being able to triumph there would be frigging great though.