I've seen it said that Rook is a better romance option for Solas because unlike Lavellan, they know who he is from the get go. So let me make something clear:
Rook does not know Solas better than Lavellan does. They know his history, his crimes, even his regrets, but what he shows when they talk to him is very much a mask.
Fen'harel is not who Solas is. As dishonest as he was about his past during his time with the Inquisition, he also came the closest to being himself ever since he took a body.
In sappy terms, he hid his deeds and plans from Lavellan, but not his heart. With Rook, it's the opposite.
Who we see in Veilguard is not some kind of "Solas unmasked", it's Solas who has returned to wearing the mask he was allowed to shed for a little while and hide the fact he'd ever worn it.
The raggedy apostate who plays mental chess with Bull, trolls Sera, beats Blackwall at diamondback, who nerds out about magic with Dorian and approves of helping every single hinterlands peasant you encounter, that's the real Solas. Keeping his past a secret is what allowed him to stop being what his service to Mythal and his people made him into, even if for just a little while.