Saul represents the best opportunity for subtly splitting the crew again. Robin, Lilith, and the other interested SHs stay behind with Saul to catch up and maybe consult some of the Ohara books, while Luffy + whatever SH may be relevant to future plots go about exploring in search of the arc's main conflict.
As Luffy & Co. start little skirmishes with the villain's mooks, like with the sumo chump in Wano, the main villain himself might threaten the SHs left with Saul (a la SHs left on Going Merry meeting mustache guy & being hit by Enel's lightning in Skypeia).
Then Luffy comes back to Saul's village, we get a CHILLING cliffhanger of everything destroyed, things are explained, and he moves on toward the arc's main fight.