It wasn't just his romance - I was kind of annoyed I couldn't play the field in this game firstly. Secondly, as you say, that there weren't a lot more points where he would dump you - that goes for all of them actually. I feel like, if Solas is so angry about the wardens, a nice big argument scene about your opinions (kind of what you get with Sera and being "elfy") and then he/you just realize it's not going to work. I don't think Solas is as entrenched in his own opinion as Sera - or at least isn't as immature - but I think there's certain things where he would be like 'no, i can't see this working realistically if you believe x,y,z and are passionate about it'.
I assume they might have done this with more time/budget. Though recently I hear many good things about the Sera romance. Seems like she is designed and developed a lot better than other romances in this game. I very often thought I would need to explain choices to Solas to deserve his positive opinion. But this happens when the weighing system does not work well. Killing a few ram and giving the meat to refugees is good, but making huge decisions he disapproves of greatly should not be balanced out by it. But I really think this is due to monetary reasons.
I often felt the same way in my Solas romance where my character wouldn't budge on her stance that all demons/spirits are bad, choosing to ally with the Templars, keeping the Wardens around, drinking from the Well, etc. I kind of wanted to see what kind of headway you could make with changing Solas' mind, the same way you kind of do with a Fenris rivalmance. But it doesn't really work with Solas, imo. He'll still tell you he respects you and whatnot. It's kind of weird, heh.
I felt similarly. My Trev did everything he disapproved of, but the small things (Hinterland side quests, a few judgements, his personal quest despite not allowing murder) were enough to just balance it out. I think a more detailed approval system would be important here but it would make matters more complicated (probably not only for the devs but also for the casual players). They tried to make it all seem more realistic by not showing us the approval rating but... well, that's just not enough.
It is okay for me if he still likes me despite decisions he does not approve of -- if I can explain myself to him, convince him of my integrity. Had they implemented that, the whole thing would be more believable.
Ohhh. I didn't even know it was possible to defer the companion quest until after the balcony. I thought the one triggered the other.
And maybe it wasn't supposed to be possible and I ran into a bug. Or I remember wrong and I got the balcony scene after killing the demon (and this was a bug). I really don't know anymore, it's been too many months now. But I know exactly that I killed the demon, was mean about it, and he still stayed with me. Because I had deliberately tried to force a break up, but exactly this girl was the only one sticking with him until the vallaslin scene even (usually I broke up earlier).