I'm interested in crowdsourcing this partly for the low cost and partly to get the vision of several designers, rather than being tied to one person's view
Let's be clear. "Low cost" and "vision" pull in opposite directions. That's nothing to do with the platform: all the well-known ones, and for all I know many or most of the rest are good at their job. You just need to be clear what that job is.
The platforms mediate clients with money and needs, and designers with time and talents.
Want lots of ideas to help your own thinking? Kick off an open contest (or whatever the terminology is for the site of your choice). Lots of people will browse, some will dash off something, a few will choose to devote more than the few minutes that a 1% or lower chance of earning your cash warrants - they'll typically be the ones with more time on their hands, of course. You'll get a result. If you're lucky you'll get exactly what you want, but most likely you'll end up with an idea you like, but not the finished result you're after.
Want a choice of vision? Then you have to invest time. Hunt around to find designers who are showing the kind of creativity you're looking for. Then invite a few to a closed contest. If you're inviting three bids, and the fee's appropriate, then you're likely to get some no shows, but the ones who turn out will invest a bit more time and energy at the first stage.