For some reason I thought I was going to be reading a really unique and refreshing take on the genre, but it was a pretty generic demon lord story. That is totally fine and probably what you set out to do (the demon lord trope minus the things that you said), but at the current stage I can't even tell the difference between your take on the trope and the many demon lord mangas that I have read. But that is fine, the premise doesn't have to be unique to make a good story.
Now this may just be a problem with me (I lost ~50 iq reading machine translations) but I found it really boring to read. So this isn't going to be proper feedback, as I found myself eventually skimming through it, and you don't really need to take what I will say seriously as I didn't properly read through your work.
I started skimming when I realised that it was all just explanations and went straight for the battles. I understand that, yes the explanations might be needed and that this is probably the least interesting part of the story, but the problem is you put it right at the start when you should be selling to the reader why they should read the book. The explanations weren't presented in an interesting way either, rather than a tense do or die situation or the protagonist doing trial and error, showing their dedication and allowing us to empathise with them, they are just given instructions to do it.
The battles themselves were also quite lacklustre. This is probably because their is really no motivation for anything and the people fighting are literal one dimensional cardboard cutouts. This is a byproduct of the way the demon lords were created, I understand that. They have no history, no real sense of purpose, just a single trait/personality given from you. This also applies to the protagonist. The simple solution would be to just introduce them later on when they could have purpose and properly flesh out their personality. Give them a reason to do things.
I honestly also think that they have stayed way to long at the palace? There is no point in keeping them there or even starting them there. What have you accomplished by doing so? You have introduced the system which you can have the protagonist figure it out themselves. You have introduced the demon lord rivals (?) which are currently cardboard cutouts and potentially ruined any tension from the rivals as they are all being used as jokes.
After writing all that I think I just wanted an edgy demon lord and action while this was too lighthearted and heavy on the explanations.
I may do an actual review if I ever read it again and I hope this feedback is okay with you even though I didn't properly read your work.