either add 'nimis' or remove the 'too'.
According to my dictionary, "sero" is used to mean "too late" (rather than just "late") by at least five Roman writers and poets - Cicero, Suetonius, Livy, Ovid, and Virgil.
What would just "late" be?
nimis sero and serius (the comparative degree) should be accepted
Is ille necessary?
No. Just "Sero advenit" is fine.