Shinano's real constraint: trained aircrew

Would a completed, more seaworthy Shinano have significantly troubled the US Navy?

Only if Japan could also provide trained pilots and sustain carrier operations; aircrew replacement was the more serious constraint.

Even a more seaworthy Shinano would have mattered little without a competent air group. The completion date is decisive because Japan still needed enough skilled pilots to form that air group. A continuing training pipeline could rotate experienced aviators back to instruct replacements, keeping operational squadrons mixed between veterans and newcomers. Stronger anti-submarine warfare, fuel security, and industrial support would also be necessary to sustain carrier operations rather than merely complete another hull.