Speaking of which, what is Spy x Family outselling currently?
Taking the most recent volume for each manga, we have:
01./ Demon Slayer (21) - 2,867,942 in 3 weeks
02./ One Piece (96) - 1,594,700 in 4 weeks
03./ Kingdom (58) - 708,296 in 4 weeks
04./ Attack on Titan (31) - 629,874 in 4 weeks
05./ March Comes in Like a Lion (15) - 571,357 in 4 weeks
06./ Spy x Family (4) - 539,783 in 4 weeks
07./ My Hero Academia (27) - 476,641 in 3 weeks
08./ The Promised Neverland (19) - 470,609 in 3 weeks
09./ Haikyu!! (43) - 465,659 in 4 weeks.
10./ One Punch Man (21) - 462,068 in 4 weeks
I didn't include Hunter x Hunter/Real and other manga on hiatus (NANA, Vagabond, etc) because their previous volume came out too long ago
I used the most recent volumes for DS/MHA/TPN even though they only have three weeks of sales because they each outsold what their previous volume did in 4 weeks. A fourth week for MHA/TPN shouldn't be enough for them to catch up to SxF, though they could switch place.
Also, both AoT and 3-gatsu have a limited edition that didn't chart in each of the four weeks, so their real number is probably a little bit higher than that, though nothing significant.
Imo, Spy X Family's 5th volume will sell a bit under 700k in 4 weeks next september. (Well, it might do even more if it receives a huge boost thanks to the talk show thing). And if it keeps selling at a similar pace every week, it will pass Kingdom with its 6th volume