A larger U-boat program invites escort production

Would Germany have forced Britain to make peace by abandoning big surface ships and building a 300-U-boat fleet?

No. Britain would see the buildup and respond with large numbers of escorts, making it an industrial and infrastructure contest Germany was poorly placed to win.

An earlier 300-submarine German program would not automatically starve Britain into peace. The Royal Navy would notice the buildup and answer with escorts. Treaty rules allowed unlimited sloops below 2,000 tons if they stayed under 20 knots, carried no torpedoes, and had only a limited number of guns larger than 3 inches. Britain was already moving toward the Flower-class corvette before war began, so an earlier German buildup could bring that response forward. A genuine technological break such as Type XXI-style boats appearing early might change the contest, but mass-producing existing designs sooner would leave Germany building less capable submarines while Britain expanded its anti-submarine fleet. Submarine strategy must therefore be measured against enemy shipbuilding capacity and technology.