Your 'fractional cxo' will cost as much as a full time junior, work very few hours, not care about your equity, and be mostly unconcerned whether you succeed or fail.
Get an employee who's dedicated to your mission and wants to prove themselves, or learn to do the job yourself until you can justify the hire.
I dont know about startup culture or silicon Valley, but i do know a fractional cfo who is more motivated and dedicated than most full time associates.
I'm not convinced fractionalness is a bad thing; i would think it adds tight feedback loop for performance that executives sorely lack.
sort of like tahoe timeshares near heavenly for periods when there is no snow.
(and your experience will all depend upon the contractor)