I think you will be good even with several hours in the game.
First for Banter Pack the first Q&A in their FAQ
was exactly that:
Q: Do I REALLY not have to start a new game?
A: Really. Although to get the Imoen/Jaheira banter about Yoshimo, you must have
had the mod installed before you visited Spellhold (and have Yoshimo in the
party for the fun.)
Then for Flirt Pack, as far my understanding goes, they add extra dialog options deep in the romance or even after the last "lovetalk", so that means you haven't missed anything.
In general if a mod builds upon the official dialogs or add individual triggers there shouldn't be a problem installing mid-game (apart from potentially missing some).