So it starts with the premise of his rebirth skill, where it sets up that he has died over and over again in order to save his friends and fight against the academy. This thread is fine, but then it just gets used horribly. So when fighting the final boss his soul gets scattered which then causes an error causing him to reincarnated 6 years prior when he got his skill (it kept him for leveling up and gaining skills past D) so then you have a pretty good setup for what the stories description gives you. A guy who has died countless times getting reincarnated and this time not having his rebirth ability, but the heroine remembers his deaths. Perfectly fine...
Then we get to where the story starts to become bad. So two random thugs show up for some reason and then he starts using them for whatever reason, which I'm not going to get into since its not really relevant. He uses said thugs to get into a dungeon where if he pretends to be dying he gets a hidden boss where he proceeds to casually get a skill that allows him to copy any skill from anyone if he quote on quote defeats them. The skill is just as a baseline designed horribly, and by this I mean its obvious that the author was trying to pull a "smart protagonist using skill" play but instead creates a skill with an exploit so obvious that if the protagonist didn't exploit it then he would literally have one brain cell. The skill lets you choose any one skill from someone you defeated + xp but you get a 1 day cooldown, but if he only does an "incomplete defeat" then it gives a random and no cooldown. As a baseline its a s*upid ability that I think ruins a ton of novels because its f*cking hard to write a protagonist who can obtain hundreds of skills and use them well. If an author adds a skill like this they need to add strict activation conditions to allow it to be a plot device without creating the issue of a protagonist who is either using there abilities to a meager fraction of their potential (Which is super hard to prevent since its near impossible for a single person to think of all of the applications of a single skill, much less hundreds) or make the protagonist become s*upidly overpowered. Anyway, you probably get my point by now... The protagonist has a incredibly s*upid skill that is just completely unbalanced.
So all of that can be summed up as the author does a very poor job of creating skills that are balanced, which is something that is required to create good world building. (And by this I mean that because the protagonist can obtain any skill, with pretty lax conditions, it means that no other power can realistically rival it) but if it was just this I could ignore it, but just the overall writing feels poor.
To elaborate its like what I mentioned above, where it feels obvious what the authors intensions are with each skill, conversation, ect. where it makes it hard to immerse yourself in the story. The protagonist himself doesn't have much of a character established. At this point we have He died 100k times for his friends -> beats up some a**holes and then proceeds to use them like s*aves -> hates the academy... thats pretty much it. The author doesn't add anything to build their character, things like when being rebirthed something like him crying in joy that they are alive (Since even if his skill makes him numb to their deaths this time his skill wasn't expected to work) or having him desire to see them, ect. Things that establish his motives, his feelings, ect. but instead we get him get harassed by a**holes and then proceed to basically ens*ave and abuse them. And most of the writing follows this same level of skill, where the characters feel shallow and poorly set up and it feels like things are set up with little regard for the future of the story.

I'm kind of tired of writing so overall I think the writing has poor world building/skill-setup, shallow character development, and a few other things that just make me think its a bad novel but potential readers might still want to give it a try. When reading web novels people generally have certain quirks that can ruin or make a novel for them and this one happens to hit the things that I dislike in a novel, but other people do completely disagree with my option so especially if you like op protag's then you might still want to give it a shot.

MC regressed after meeting the 'last' boss. MC decided to amass different powers after losing his previous restriction. MC become arrogant and challenge everything heads on even after losing his previous regression ability. MC faced defeat after learning the world's truth and decided to continually regress after regaining his regression ability. end