Melpy
I'm very sorry but I don't know what advice to give, as I said in one of my previous comments I've never written fanfiction or novels. The only advice I could give you, seeing how this book will surely be abandoned, is to write another fanfiction but planning the events in advance, like writing a lineup with the various events. I apologize for the uselessness of my advice, but I don't know what else to tell you, I really like how you write and I would like to read your other stories, so even if you abandon this fanfiction I ask you to create another one by structuring it
Idk if u read old comments but ur right that u made mc too op u could correct this by making the power up partial meaning he only gets some of them like from muzan he gets his immortality and demon making and nothing else. U can also add slight weakness to heat/fire/sun as it will be good to see mc trying to counter this but becuz he is immortal the dmg he takes his body recovers but it causes pain etc(unlike muzan). And zenitsu's swordamanship by giving him only the 1st form
The only advice u have is getting your story pacing right, cause as you said mc is already too op for aot world. There are many low power abilities and techniques in manga and anime you could have used instead of zenitsu and THE MUZAN, like for example the Rokushiki martial arts style that marines in one piece use. In all honesty as it stands, write a new fic, or start over from zenitsus template with a less op power. Aot does have potential with the titan shifter powers and its lore, but ur mc just became too op.
It truly is hard to give advice on something you feel doesn't need a change. I feel you were doing fine with letting him do as he wishes, basically and because of the not so high power levels in aot you could use this world to stockpile templates then maybe world hop to do something else. That's about all I got. I don't think this will help, but for what it's worth, it will be here. Please don't drop. Keep up the good work.