הִנֵּ֗ה עוֹדָ֛ךְ מְדַבֶּ֥רֶת שָׁ֖ם עִם־הַמֶּ֑לֶךְ וַאֲנִי֙ אָב֣וֹא אַחֲרַ֔יִךְ וּמִלֵּאתִ֖י אֶת־דְּבָרָֽיִךְ׃
Ecco, mentre tu sarai ancora là a parlare con il re, io entrerò dopo di te e confermerò le tue parole».
Rashi
And it will come to pass when your days are finished to go with your forefathers That is to say, do not fear that I will hasten the matter, but when your days are finished, then it will come about, and I shall raise up your seed after you. and I shall establish his kingdom And so he says of Solomon, (I Kings 2:12): “... and his kingdom was firmly established.”
Chizkuni
וימלא אחרי, “he followed Me fully;” he completed Moses’ task, My messenger’s task, trying to make him more effective. The use of the root מלא in the same sense as translated here is not unique; we find an example of it in Kings I 1,14: ואני אבוא אחריך ומלאתי את דבריך, “and I will come immediately after you and confirm and reinforce your words.”(The prophet Natan to Batsheva, mother of Solomon)