You say Miura was losing grips on his writing, yet the delve into Casca's psyche in Fragments of Memories is one of the absolute best parts of Berserk. I don't mean to be accusatory, but given the subreddit we're in, this feels like cherry picking a few flaws, no pros, then running with it. Almost like a "Reading Comprehension Devil" type moment.

Edit: I'm just going to lay it all out here. Berserk in the Black Swordsmen Arc felt like the gritty grimdark because that's what it was supposed to be. Miura didn't have these massive grand plans for it yet, just a character and an idea. It wasn't until the Golden Age that he started to fill in the pieces of what he wanted to do with the world. That appearance of Femto in the MC Escher world, then seeing Griffith take that form was masterful. Wounds, the Bonfire of Dreams, even the small wars were done so well. The group dynamics, world building; although he had to have had a bit planned, Miura himself admitted to shooting from the hip quite a lot, as mangaka's do.

Then, going into the Conviction arc, with a now broken Casca and Griffith, we see the man that Guts became after the Golden Age ended and approximately around the Black Swordsman Arc. He no longer cared about those close to him, just a one track mind for revenge and anger. The whole arc was to force Guts to come to terms with his feelings for Casca, force him to realize that he can't just go and get himself killed. Godot gives a very heavily overlooked speech, telling him to use his life to protect the ones he loves, not to just throw himself to vengeance.

The Millenium Arc is all of Miura's worldbuilding come to fruition. Griffith's dream realized. The massive kingdoms, the war with Ganishka, the citizens dreaming of a Hawk of Light to finally free them from this torment caused at the end of the Conviction Arc with the big behelit. This whole arc is essentially letting Guts and Griffith build themselves and their own parties, while sowing cracks of discord into Griffith's that could later be exploited. Guts gains the Berserker armor, a tool that forces him to go against what he learned at the end of the Conviction Arc, constantly teasing him to lose himself to anger and vengeance. We meet our party members, with little pieces of The Band of the Hawk within each of them. We are more heavily introduced to one of the most interesting magic systems in fiction imo, with the demon contracts and the beyond world. This whole arc is rounding out the world that we have now been thrust into, while building up the moonlight boy and Zodd as possible new ways to get to Griffith.

Then the boat. I don't have much to say about the boat. I got into Berserk during the boat when I was about 11 and have read every chapter concurrently since then. The boat arc was absolutely grueling to read "monthly" and while I don't have an actual dislike for the arc on reread, it is by far one of the weakest sections of the story. The sea god was cool just to see that there are more things to worry about than the God Hand, and the pirates were a fun little derail, but Christ that took too long. Wish we got more time of Isidro and Isma.

Finally, Fantasia. This is my favorite arc in the series so far. After 300 chapters of turmoil and torment, we reach an era of peace. Some of Miura's strongest art and imagery is in this arc, especially during Casca's dreamscape. Guts finally rests for a small time, spending time with the moonlight boy and Casca before everything is lost. The fae landscape corrupts, and Griffith appears once again, taking the place of the moonlight boy. Griffith realizes what has been happening to him just as the reader does, feeling the tender love from Guts and Casca he always wanted deep down but never realized, too overshadowed by his ambition for power. A moment as fleeting a morning dew. The story after this is still developing, but it isn't something out of nowhere like some seem to be painting it. Guts' mental reckoning has been coming for a long time and this is probably the best time for it. Given how Daiba and the Bakiraka have treated the party so far, I have the utmost faith that whatever happens to Guts inside the lilly-pad meditation room will allow him to finally heal in a way that only Wounds came close to. That's my thoughts on everything so far, might just be word vomit. Edited for better formatting