for $10k-$15k and good brief, you could get let's say 5–8 freelancer designers to do a PAID creative pitch (there's your difference between spec work and 'normal' creative pitch), get multiple high quality (and relevant) options, and give the job (and the rest of the money) to the designer you like most.
Basically, you pick few designers, who you think will fit the job most (looking through their portfolio) and will pay them for the time pitching to you. That's it.
Basically, you pick few designers, who you think will fit the job most (looking through their portfolio) and will pay them for the time pitching to you. That's it.