Because, going off of the pre-skip blueprint, Blackbeard comes last, at the end of great war, thats why Kizaru is next and because he has unfinished business with Zoro since Sabaody and is the only admiral who can keep Zoro in check since light is the most hax among the Admirals.
Gandhi has to come right after him since both will fall in the same arc, Shiryu's time then right after those two. 3 opponents in one arc is doable for Zori. Also, its not too soon, we are already in the end game and Luffy is about to obtain 4th RP the very next arc and from there he needs only to learn the history before he goes after WG.