Any barriers you introduce in between the user and value of your service (even if that value is just a consultation with a human) will necessarily cut down on conversion. The question really is: is the cost of losing (optimistically) a third of your new leads worth as much to you as (potentially) weeding out clients who aren't serious?
Also, are you considering all use cases? What about a client that's totally serious, but hasn't picked a timeline for their project yet? They will hesitate at that date picker, and possibly fall off right there. Unless you're currently spending tons of your time filtering out non-serious clients, I think simpler for the user is the way to go.
So I'm thinking of thinning the form down to Name / Email / Tell me about your project / budget (adding a 'not sure' option). Better to get people I don't want filling out the form, than nobody filling out the form at all, I think.
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