The listed cars span quite a range. It sounds like ride quality is more important than handling, so sports cars are out.
If you want a fun car within the Lexus line, the IS would be a better bet than the LS. I've never driven the IS C hardtop convertible. Avoid the 250, not nearly enough midrange power.
Infiniti offers a sportier car in the G37/Q60 hardtop convertible. Less prestige, and probably a firmer ride. But more stylish, and discounts tend to be large. With both the Lexus and the Infiniti I'd be a little worried about the complex tops if this is a car you plan to have for a while.
Mercedes tend to have the best ride among the German manufacturers, so the E could be a good fit. Reliability is a bit of a question mark, but no more than with the Audi you already have. I'm not a fan of the styling, but this is subjective.
To make more specific recommendations, it would help to have a clearer idea of what you're looking for. What ruled out the cars you've already tested, especially the A5 and 335i? If power or engine refinement was an issue with the A5, the S5 might be a better fit.
What cars have you really liked, and what did you like about them?