I think romance wise, so long as he's like his show self; Harwin strong is the better marriage candidate. If only because the thread really wants him in Rhaenyras harem (and to be honest, unless there's a good bit of skullduggery going on; Harwin, much like Bobby B likes a warrior woman, but thanks to lack of options, settles for show Rhaenyras spunk)
The challenge is, that you'd have to marry him, or all of his children will stick out like sore thumbs.
A better pairing for Alicent (if we also plan to seduce her, and potentially even Mysaria) is Laenor.
If only because he can do the honerable and knightly things that Alicent likes in men; and none of the seemingly unsatisfying sex (if we go with the idea that's she's closet Rhaenyra sexual/Rhaenyra leaning bisexual)
They don't have to get pregnant, and if necessary, you could always use Harwin, since dark hair and brown eyes are already Hightower traits.