I am not trying to change this view but add...
I think even if Geralt is trying to focus on Ciri, selflessly in fact, he SHOULD have to face Triss and explain it. Triss is a STRONG women but, there is a history with Geralt somehow always sliding back and away from her. How could she not be a little (i hesitate to use insecure) reluctant to believe this won't end up just like 6 months ago.
Triss might try to get close and Geralt would have to call her down...the hurt he sees in her face would wash over him and cool his head. Stepping in close he would take Triss and whisper to her:
"I'm sorry, right now I have to focus on Ciri...I promise, when this is over and she is safe, you won't have to wonder where I am or what I am thinking ever again."
Something like that would then explain why they share so many glances or looks...and so (frustratingly) few words in the last Act.
Yeah i don't mind the focus being on Ciri, but if we are looking at this from a game pov, we don't see anything and in the end we start making our own fanfiction/headcanon because Geralt is way too preoccupied with Ciri, not that in itself is a problem but there is no time or space for Geralt to interact with the other special lady in his life. He doesn't reassure her (like you described), but Triss shouldn't feel like she is playing second fiddle, when the opportunities for Geralt to be closer to her are very obvious. I kind of almost looked at it like Triss is accepting defeat in that she has Geralt and has a future with him, but can never really get close to him or have him open up to her the way she wanted.
Yet in those scenes in Novigrad, Geralt is more than willing to open up and get close to Triss, so then we get to a point where Geralt forgets about his relationship with Triss and puts it 'on-hold' and Triss comes to the same conclusion and before long, the game ends.......
Geralt: Hey Triss, sorry we haven't had much time lately
Triss: It is fine, lets make sure Ciri is safe, and then come back to me
Geralt: When this is all over, we will have our happy ending in Kovir
Triss: I look forward to it....when it is all over
(Geralt gives her a kiss, with Triss returning the warm smile she once held for Geralt back at the lighthouse)....and that would of been a quick scene reassuring Triss about what their future together means for Geralt.