I'm tempted to put a copy of my original post under suggestions and feedback, but this is why I put it here, besides that I didn't see that section lol.
IMO saying "I love you" can convey so much more than that you're absolutely committed to the relationship and will never leave. It can convey that you want what they want, not just because you're passing on the choice to them, but because you are so on the same wavelength that you literally feel exactly the same way they do and your decisions match up.
In spawn astarion's case, it could be perfect for conveying that your relationship isn't just a manipulation like all the flings he tricked under cazador's command and for getting him to admit to himself the truth of his own feelings. You chose him of your own free will and he chose you of his, and both your feelings are real.
"There's nothing I want more" would also be a great option, but I think you're underestimating the power and meaning of the words "I love you."
