we still don't know how long Loki was happening, since it wasn't in time. Wandavision was about 10 days to two weeks after Endgame, putting it in October 2023. Loki escaped from 2012 while the October 2023 Avengers were trying to get the Space Stone. We don't know what time he went to but it's clear that the Space Stone can travel in time and the TVA doesn't abide by time rules. Sylvie created a bunch of multiverses in 2050, the episode before they nearly died on Lamentis-1 in 2077, but they were saved by a variant signal. However, this seemed to be the same day to the TVA. Then they met He Who Knows right after being banished to the end of time. So we don't know when the multiverse was opened, or if its time of nonexistence was just erased entirely and now it's as it always existed (explaining why Kang instantly had overtaken the TVA). It seems by SM:NWH that it already exists (but this is months after Wandavision, as Far From Home happens in July 2024 (actually eliminating Doctor Strange from the list of possible suspects here).
So the only suspects now are Sylvie and Wanda. Sylvie directly killed the guy keeping one universe intact, making her the direct one to do it. Wanda breaking reality couldn't have made more multiverses, but she probably can travel between multiverses. Let's just hope there are no evil Wandas or Ultron/Wanda hybrids, that would explain the Darkhold prophecy Agatha told Wanda
(I've only watched loki once tho, might need to refresh my memory on some things)