Part of me will always mourn that we didn't get to see Chat Noir fighting Monarch, his father, in the finale battle. That's always gonna be nagging in the back of my mind.
At the same time, it was a little predictable, and what the writers chose to do instead makes a lot of sense when you think about it, and is maybe more satisfying for Adrien's arc regarding his relationship to his father than if he'd had to fight Monarch knowing it's Gabriel. (Not talking about the whole "he still doesn't know his father was Monarch" bit, I'm strictly talking about the fights here - that one's another topic entirely)
I say "predictable", because of what happened in "Representation". Adrien got here his moment to fight his father, as a seemingly akuma victim, and this on his own without Ladybug.
With this set-up we have here: Adrien confronting and fighting his father as in Gabriel Agreste, and Marinette confronting and fighting her nemesis as in Monarch. That's always what we envisioned as the finale fight against Monarch would be: Adrien vs his abusive father and Ladybug vs her enemy. It just didn't happen in the same fight, so both could separately have their moment and their thing to say.
I think in the end it's significantly more important that Adrien got to fight Gabriel Agreste as Psycauchemardeur focusing solely on him as his father, than have him fight Gabriel Agreste knowing it's Monarch. Because everything that happened in Representation, everything Adrien told his father as Chat Noir and everything Adrien was angry about regarding his father's behaviour with him, that all comes down to how Gabriel has acted as a father to him independently of him being a city terrorist.
If Adrien had had the realisation that his father's actions were terrible through the discovery that he was, on top of an abuser, a supervillain, then the whole "terrorising innocents" would have overshadowed in Adrien's anger all that Gabriel did to him personally as an abuser & shitty parental figure.
What I mean is that Adrien realising his father is shitty independently of knowing he's a terrorist and confronting him about it in a fight is part of his arc and is more impactful than Adrien confronting his father for his supervillain acts. He didn't realise Gabriel was a terrible person because he was a supervillain, but because he understood he was an abusive father and that's amazing
And that's why he got his moment in Representation while Marinette had hers in the finale - Gabriel is a shitty person both as a father and by being a magical terrorist. Adrien confronted him about the "shitty father" bit, and Marinette about the "magical terrorist bit", and both these things make sense with their respective character arcs & respective relationships to Gabriel
