The 76mm Sherman wasn't half finished, it was completed and ready. The Army just rejected them because they valued their 75mm HE shell too much, and didn't consider better AT a worthy trade because they used Tank Destroyers.
You're misinformed and seek to misinform others.
M4A3 (76mm) tanks went into service in mid August in two companies in 3 tank battalions. 70 went into action with the 7th army which was coming up after the landings in southern France post August.
During Operation Cobra, aka the breakout from Normandy 102 M4A1 (76mm) went into action as part of 2nd and 3rd Armoured Div's.
By October 1944 entire Armoured Divisions being sent to Europe such as the 9th were full equipped with M4A3 (76mm) in their battalions.
By the battle of the bulge (December) 1/3 of the US ETO forces were 76mm armed M4's. In armoured divs 2 times the number of 76mm M4 were available whereas the Infantry Div M4 battalions and independent Tank battalions had to make do with the 75mm gun M4s.
On February 15 1945 the 12th Army group refused to issue any further 75mm M4s resulting in around 600 M4 (75mm) sitting in depots.
The Idea that US forces only used 75mm Shermans relies on ignoring US armoured Divisions from the Normandy breakout onwards and pretending the only tank battalions were the independent units and integral Infantry division Sherman(75mm) battalions. It's as disingenuous as arguing German "heavies" were in the minority while ignoring Panthers making up 46% of the Panzer arm in the west.