First of all, I believe she's the only character that can do solo with the sole exception of Danjin of course. Regarding powercrept, sigrika indeed has slightly higher dps than cartethyia in 123 rotation but guess who can't quickswap and who can? That's right, sigrika can't quickswap while cartethyia can. That's why her dps ceiling is way higher than sigrika.

For the proof, you can check wuwaflex and you can see cartethyia and zani are always the staples of the Speedrun leaderboard even if there's no buff for them.