Hi there. I personally haven't played through trying to romance Minthara - but what I can say; is that this is true for almost every other character. You get your 1 scene in Act 3, followed by essentially nothing but an ending title mention. Of course; you're able to kiss them, but aside from that, the game is dreadfully lacking in romance content post Act 1. It's true, this isn't a dating sim game, but Act 1 really has you thinking you're going to get a lot more content with everyone throwing themeselves at you every long rest.
Really wish they woulda kept that same energy in Act 2/3 because as it stands, it makes you feel like all the romances are just a bit.... lacking. Like; sure, they all have pretty good individual character arcs, but aside from the 1 scene you get in Act 3, along with the druid Orin Circus scene, it's barely noticable.