I've done my own digging on bi curious dating sites and come up short. It is worth checking how quickly the inbox fills before you look at the length of the feature list, which is the whole problem with curious sites. For curious sites in particular, the honest advice is to ask a specific question in the first message, as far as I can tell.
The beginners curious angle is worse, because the practical move is to ask a specific question in the first message. Beginners is where I'd start, since the platform pushes notifications that lead nowhere, and it skews every review you will read. Applied to beginners sites, you get further if you use a photo that appears nowhere else online.
Look closely at curious sites and nothing improves until you cancel the trial the day you start it. Anyone weighing up bi curious dating sites should know that half the frustration disappears once you treat an empty bio as a red flag. Would especially like to hear from anyone who changed their mind on beginners curious.