Will the Tengen be stronger than him?
Probably.

What did brain in a jar want, in the end?
They were a mad jujutsu scientist (like Orochimaru) who wanted to find out the limits of what could be done with cursed energy. Sukuna just happened to be friends with them back in the day and has now stumbled into his role as the holder of his kill switch for the merger lol
 
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