Dont worry so much an just enjoy the ride. You also dont have to be evil with Astarion. He is more "we bullshit our way through Faerun, also I like naughty and hilarious stuff, and murdering someone on occasion" (lets face it Faerun has quite a lot of people who need some murdering.
He doesnt like promising to help people sure, but usually has not much problem with a "I look what I can do!". My first PT was a goody two shoes elf who helpled everyone and was nice to kids and I had no problem getting his romance and max approval at the end no problem.
Just allow yourself to be a bit chaotic at times.
At some point after doing some main quests there is a sure chance to start his romance, you will know it when you see it. And if you already have fair approval you should be safe to get that I think.
