Alright finally completed it, actually thought that this one will take me a month but surprisingly it took me only a week for its rollercoaster pace.
20th CB started off excitingly, a gripping pace with titbits from past to keep the readers engaged. Imo, the concept though may sound silly(it's childish anyway) was brilliantly executed so that the readers are constantly drawn in to search for the answers and clues. This continued till the death of the first "Friend" and the manga's quality was 9/9.5 until therethere, but Urasawa drove himself to a corner by the introduction of a second "Friend" and never quite brought the same quality back again. The only thing that kept on ticking is the mystery of the second "Friend" whom many people believed to be Fukubei and some questions unanswered, but the problem is that the readers are blueballed ultimately. And yes I know very well 21st CB explains that, but there are clearly not enough foreshadowings in the main series and one of the hints just feel shoehorned for the sake of it.
Another problem with the last half is that Kenji got bailed out quite hilariously by the power of his musical charisma(he would have been dead 3-4 times tbh), characters quite magically popping up out of grave which was quite expected(poor Mon chan tho), and introduction of new characters which clearly no one had interest in, like that of DJ. Also yeah, flying saucers and what not lol.
Now talking about the characters, perhaps I actually found them the strongest point of the series, with the exception of Kenji's character in the last half as I said. No doubt it matches his aspirations as a youth but was too convenient. I really liked Kanna's character, perhaps my favorite of the series along with Otcho, and most of the main cast was pretty great too.
I liked how the ending played out with Kanna even though it was underwhelming. Also Idk if it was only me, but some of the plotlines were quite predictable to Monster and Pluto, but more importantly, I called the identity of the First friend a long ago in the "Fukubei" chapter. I mean come on, that cheesy overdramatic confrontation, the golden rule of "not dead if body is not checked" and a number of clues gave it away for me.
I am sure Urasawa himself knows he could have done the ending better, maybe that's why he made a new ending a few months ago with its new release(still waiting for anyone to translate it).
Overall I am conflicted between 8/9, I will give it a 8 now since I gave 9 to Monster which is overall a more polished work imo, and maybe change it to 9 afterwards if I feel like it.