I high five you on this! I regret going back and siding with the mages to get Cullen's extra quest. I wish I had stuck with siding with the templars. It all feels completely off. This is not how I intended to role-play for this particular character, the only one I intend to romance Cullen with. I honestly think he has enough past burdens and I don't feel the need for that to be added to in my game. Plus, it just goes against my character, siding with the mages. I am already head-cannoning that this decision is something she agonizes over and continually asks herself if she has made the right one because she is horrified to see what has become of the templars. It is almost a bitter feeling for her, that she took the time to save a group that stupidly sold themselves into venatori servitude and then to see the people that she had originally intended to ask for help be turned into monsters. Is isn't like she knows what would have happened to the mages if she hadn't gone to Redcliffe so when I head-cannon it is hard for her to get past all of this. I don't know, I guess I am stuck with it now, lol. I have restarted twice already and I am not doing so again.
I also love how much more 'light' his romance comes off when siding with the templars. We have seen this guy go through hell and back and it was really good to see him be able to start to work through it and just be happy that he has finally found someone that he deems worth holding onto, someone worth trying for. I like my happy laughing Cullen (I love how much you can get him to laugh), I like seeing him slowly letting everything go after all that he has gone through. There were two things I always hoped for: that he had a happy childhood and to see him start working through his lyrium addiction and what happened to him before the Inquisition. I have those and for me personally, the nemesis quest is added angst that I don't need and don't want to have to see Cullen go through more than he has already had to endure.
I really do commend Brianne, though. She was so thoughtful in his writing. To be able to make his romance so it was able to go in slightly different directions, although subtle, it is nice to see that everyone can be happy with it for different reasons. There is enough leeway to be able to interpret things differently. It is like she always had us in mind, knew that we all had personal preferences when it came to head-cannoning and worked out a way to not stomp all over that.