I wouldn't say Triss specificly brews something behind Geralt's back to oust him but I think in the first instance she follows her own path for personal benefits. She has good relation to wealthy merchants with influence, advised the king. I can't imagine that a woman like her is able to care (even love) for one ordinary person.
But I didn't like Shani either. I mean, I don't want to say I can't stand both. Shani is imo too pragmatic and too possessive in regard of an ordinary person.
I wouldn't call geralt an ordinary person. he is a witcher, a mutant. A special person with no ordinary tasks.
Triss has also more in common with Geralt then Shani has. Triss will also mention that geralt once had a relationship with an other sorceress (yennefer). She is friends with that other sorceress.
I always wondered what happened with yennefer, geralt's true love. Maybe he returns back to her after the last fight, but then again he has amnesia.
Triss would fit more with geralt then shani would, because geralt had already a relationship with a sorceress. Next to that Geralt is no common person and Triss does also no common business.
Shani is just a common medic. There are also enough references to Geralt and his relationship with sorceresses. For example; a noble townswoman in the new narakort ask geralt if he is only interested in sorceresses and not in other people.