Surface raiders would not free the U-boats

Would sending all German surface raiders out together have given U-boats free rein against shipping?

No. British heavy forces would hunt the raiders while convoy escorts continued anti-submarine work, and only a fraction of the U-boat fleet could be on station at once.

British cruisers, battleships, and heavy hunting groups such as Force H would pursue German surface raiders. Sloops, corvettes, and destroyers assigned to convoy escort would continue that work. A coordinated German sortie might become feasible around 1942 with substantial supplies and government support, but it would still face serious difficulties. U-boat totals also overstated the number available in the operating area: the rule of thumb given is roughly one-third of the fleet. Sending the surface ships out would therefore leave a convoy screen and would not put every submarine on patrol.