Reserving Felix until “Emotion” was a good call.
I think the expectation for him to be making Sentimonsters like Gabriel was making villains to get the Miraculous... would've been too repetitive, especially when Season 4 and 5 were mixing up the formula more than ever.
Frankly, Emotion being in the third act of Season 5 makes him a delightful surprise after his last appearance in "Multiplication."
Furthermore, him creating a well thought out scheme such as slipping into a high society event that Adrien will attend and creating a Sentimonster that will allow him to snap all he chooses out of existence shows that he didn't rush into things like his uncle had.
Rather than try again and again with some new creation for Ladybug and Chat Noir to beat up, he bides his time for one big gambit.
It probably helps that if Monarch caught wind of him in Paris (his Miraculous would likely have a limited range to detect and harvest emotions), he'd be wide open for his uncle as well as Ladybug to get his Miraculous taken away.
His Heel Face Turn also works since, well, what is he doing but becoming the very things he hates: his monster of a father who'd snap his fingers and puppet his son out of spite. Finding that Marinette wasn't coming back in his care for Adrien, he had a chance to take a hard look at himself.
He’s something of a Wham Episode of a character if I may show my inner Troper. Basically he comes into an episode and doesn't exactly stick around for the episodic shenanegins we usually get with his episodes often concentrating a lot of myth arc revolving around Adrien's family.
He's also popular because he also doesn't overstay his welcome, making his appearances rather exclusive. The "absence makes the heart grown fonder" phrase comes to mind.
Honestly, I'm surprised by how well they made it feel not at all rushed but rather a Trap Card they were waiting to spring on us. I do think they should have Argos be Ladybug's more morally ambiguous ally, especially as it concerns Adrien.
