It could be any one of several things, but I think the fuel pump is the first suspect. There is a check valve in the pump that is designed to maintain residual fuel pressure when the engine is not running. With age and wear, or even a piece of debris stuck in there, the ball will not seal the seat and the fuel pressure will creep off as it sits.
(No, there is no check valve available separately and one would not do this anyway since changing the pump is a bit of a job requiring removal of the fuel tank.)
Next time try turning the key on for a few seconds, back off, back on, etc. about half a dozen times and then try starting it. If it starts right up then you know the lack of pressure is the problem.
If you’re overfilling the tank, stop. It’s ok to round it up to the next dime or whatever but do not continue to force feed it gas as this can affect the canister system. Hope that helps.