ok wow, I never saw this message in my inbox until now (you posted it a month ago apparently), so sorry for the super late response x__x;;;
I think Thomas was fairly indifferent towards Susan. Mostly it’s because he just never had much time with her, he didn’t get to know her the way he knows all the other important people in Harry’s life. It’s always bugged me that the fact Thomas threw Susan to the vampires in the first place was never addressed. Did he ever apologise? Did he ever feel guilty? What did Susan think of him? By the time Death Masks rolls around, they seem ok for the brief moments they’re around one another - or at least, there’s no angst or awkwardness - but I can’t help thinking there should be some bitterness on Susan’s part and guilt on Thomas’s.
I also think Thomas is best around people who can bounce off his humour. Molly’s great at that due to her peppy attitude, and I really like the dynamic between Thomas and Murphy - they have a pretty strong platonic friendship at this point, which is thanks to how they are both ride or die for Harry. I think with Susan, things would have been a bit weirder maybe? Because Thomas would likely be wary of how he acts around his brother’s girlfriend, and likewise Susan would be on guard around Thomas’s predatory nature, whether or not she was part vampire. I really think any joke Susan makes about finding Thomas attractive would fall flat, specifically because she’s already in a relationship with Harry, and it would be awkward as hell for Harry to overhear that kind of talk - we know how serious he felt with that relationship. Maybe if Thomas & Susan hung out more it would have become a more natural part of their banter? But knowing how sensitive Harry was/is about his relationship with Susan, especially after what happened post-Grave Peril, and post-Changes, I just can’t see it.
Also Susan’s experience with her Hunger is very different to Thomas’s, so I’m not sure if it’s something they could bond over either. Susan has magical coping mechanisms and a support group, while all Thomas has to control his urges is his own willpower and the memory of what he did to Justine.
I feel like I’m being a bit uncharitable towards Susan here, though. Like you, I’m not her biggest fan–IMO she’s too defined by her relationship with Harry in the earlier books for her to really come into her own, and she’s too much of a generic noir genre intriguing/sexy love interest character to develop unique quirks that would allow for interesting interactions with the more well-developed characters in the series.
Overall I don’t think Thomas would hate her, nor would he love her, but I think indifference is too strong a word. He’d probably care about her only because of her relationship with Harry. Susan is someone Harry truly loves, and Thomas would wholeheartedly respect that love, but he would not stan the ship the way he clearly does with Harry/Murph.