So, I'm about to romance Theron for the first time as an Imperial Agent. I've romanced him twice before, both as republic-side characters, so this is something new to me.

My question is, as an IA, do you have to flirt with Theron right from the start or can you begin by 'hating' him and not trusting him, and then start flirting when you get to know him a little? It'd be a bit jarring for my roleplay to have to take all the flirts, especially at the start.

Which is the key flirt that you have to take with him to allow you to romance him? Is there one? I need to make sure I don't miss it while also being realistic with my character's personality!

If you want to romance him already before Kotfe and have the romance active during Yavin you should at least take the flirt when you are about to leave for Yavin. The one after the meeting with Satele and Dart Marr - Lana and Theron will ask you if there is something else and you can say you want to speak with Lana or Theron first or leave to Yavin without talking with them.

You don't need to flirt with Lana or Theron before that to be able to take this flirt as far as I know - because I never flirted with Lana and had still the flirt option with Lana too. But I would recommend to take at least the flirt option when you rescue Theron - not that it is necessary but I think it fits to show some development between PC + Theron before you have then the flirt with kiss. Otherwise it would come really out of nowhere.

If you don't take this flirt there is no romance dialog on Yavin but you can still romance Theron in Kotfe in chapter 9 - there you have to take the flirt option in the cantina to speak with Theron and then you can start the romance there.