Congratulations!
While it is possible to 100% the game without playing 9.0 more than once, I would recommend you return to it if you want to farm custom specials quickly later. *However*, beating Master Core isn't really necessary to effectively use 9.0 for farming, you can select give up when you lose and keep most of the things you found. And it's less stressful to use it for farming purposes just in general, whenever you lose you can just give up and keep most of your things and it's NBD.
9.0 isn't the only way to farm customs though, there's Smash Run and other things too. 9.0 is just probably the fastest.
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SonicMario
: DK is a total beast in single player modes.