In a new hotfix today, Blizzard has made another set of changes to BFA raids, this time around making tweaks to Mythic N'zoth and Mythic G'huun, making both fights soloable.
Other QoL Changes to Solo BFA Raids
You may use your Heart of Azeroth to restore sanity as many times as you want, with a 15s cooldown. However, when testing, the Sanity restoration wasn't working.
Reminder: A hotfix earlier this week made it so that you no longer go insane on N'Zoth if you don't have Ashjra'kamas, Shroud of Resolve equipped!
There have also been hotfixes done to the Bwonsamdi part of the encounter, but we were not able to test it, likely making its DPS check non-existent.