Isn't that roughly what Saguri-chan sold? Also, is World's End Harem the top-selling series currently running in Shonen Jump+?
Yep, its the best selling both in physical and digital. Its digital rankings also outdo many of the main magazine's series. This is the list of series in Jump+ that rank on Oricon:
*1│*70088 (3)│138565│162482│******│185038┃185,038 (*31)┃2018/02|終末のハーレム 5
*2│*51926 (3)│*94466│114962│129262│------┃129,262 (*24)┃2018/02|ダーリン・イン・ザ・フランキス 1
*3│*34020 (4)│*57733│------│------│------┃*57,733 (*11)┃2018/01|花のち晴れ~花男 Next Season~ 8
*4│*20392 (3)│*39804│------│------│------┃*39,804 (*10)┃2017/06|ヘタリア World☆Stars 4
*5│*13098 (3)│*26003│------│------│------┃*26,003 (*10)┃2018/02|カラダ探し 17
*6│*21502 (4)│------│------│------│------┃*21,502 (**4)┃2017/11|食戟のソーマ L'etoile -エトワール- 5
*7│*19570 (7)│------│------│------│------┃*19,570 (**7)┃2017/12|青のフラッグ 3
*8│*18397 (7)│------│------│------│------┃*18,397 (**7)┃2017/12|阿波連さんははかれない
*9│*12342 (3)│------│------│------│------┃*12,342 (**3)┃2018/02|れっつ!ハイキュー!? 5
10│*11468 (3)│------│------│------│------┃*11,468 (**3)┃2018/02|ヴィジランテ -僕のヒーローアカデミアILLEGALS- 3
Darlifra might possibly overtake Harem with volume 2 but its also possible that vol 1 was simply a novelty and people are no longer interested in the story. I'm sure there will still be buyers for Yabuki's art and "additions" but I don't know whether everyone who bought the first will buy the second.