Could privateers replace a French battle fleet?

Would France have fared better with privateers and smaller powerful ships instead of trying to match the Royal Navy's battle fleet?

No, because privateers could not break the blockade, could not be directed reliably, and could not perform the fleet's invasion-support mission.

The Franco-Spanish fleet at Trafalgar had an operational purpose beyond commerce raiding. It needed to release the force at Brest, combine against the Western Squadron, and create a path toward the Channel. France did use privateers at several points, but the state could not direct them like naval units because they followed profitable targets. Royal Navy blockades also made it hard for raiders to leave port and return with prizes. A battle fleet at least had a chance to contest the blockade, while dispersed privateers could not substitute for the concentrated force needed to support invasion plans.