Yeah I mean Thanos can't be killed in the conventional sense because Death won't let him die. I figure thats the only way he'd be a match for all the heralds at once.
As for Strange not being able to control all that power thats not true, Kaluu only said as a consequence of him occupying a plane with such highly charged energy or whatever his power would unconsciously destroy galaxies if they just ported to earth's dimension.
Strange lost his ego and wasn't really Dr Strange or Shuma at that point he was just a being of power who could do anything.