Khiona’s stat line is almost comedically average: 739 / 708 / 1336. The average dark five star hero is 729 / 701 / 1357 (not updated since Seshat came out, but it shouldn’t have changed a ton since then). So she doesn’t immediately have a huge strength to boost.
What I’m thinking about is your usage right now versus in the future. As is, her talent is kind of wasted at tank, where it helps 0% against the tiles that will likely kill her. Boosting her Def/HP is the only way to improve her tank performance.
However, once you slide her to flank you’ll probably want the Atk path, since Evade will be more of a factor in her survivability (I also like that she can buff Joon from that position).
Finally, from the raid teams you posted I think her special skill direct damage is an important part of how you will inflict damage, which makes me again lean Atk. That’s how I’d go, ultimately and live with the less than optimal tank stats (which is probably the case even if you boost her Def/HP).
In calling Khiona’s stats ‘average’ I am by no means denigrating her. She’s good. Stats are only one part of a hero. I once posted that Evelyn had average stats for a five star nature hero - which is true - in a thread about her emblem path. Does’t make her not good.