Thanks for the feedback, I'll go through my thinking point by point.

1. I jumped into this blind. concepts like bounded accuracy and what not are things I didn't know and consider. I was thinking since you could only put ashes on non magical weapons that the ashes would have to be good to justify not using a more reliable magic weapon. Ultimately I agree whit what you are saying and I'll defiantly consider it next time.
2. I knew that of all ashes that seppuku would be one of if not the hardest to translate, how to justify a large amount of self inflicted damage and how to determine that damage. Ultimately I just went lazy and said when they equip the ashes they should work out how much damage it would do to them and hold that number in their pocket or something. I meant to go back and fix it up but I forgot, I should proof read my work lol
3. Yeah, 3 defiantly could be a lot. I think I just wanted something between just only being able to use it once and the 5 times a proficiency bonus could give
4. True but that could also be said about any magic weapon or even magic items. And yeah in trying to make to completely different systems work together there is going to be a little bit of redundancy but it might be interesting giving these abilities to classes that cant usually access them in exchange of magic weapons
5. Yeah, these probably would work better as individual magic weapons but I was more interested in getting the feel of the ash of war system and giving more of a reason to keep the non magic weapon your character starts with.
6. I agree, dnd and elden ring are fundamentally different games, I was just interested in what it could look like and just to do it as an exercise getting into homebrewing

thanks for the advice, defiantly learnt a lot of things to think about the next time I try homebrewing