For grimdark I recommend The First Law trilogy. It's a little like Lord of the Rings mixed with Game of Thrones. You won't find too many fantasy creatures or instances of grand magical displays, but the few you do will be pretty memorable. The characters are all terrible people and I love it that way.
An honorable mention is The Warded Man which has a world overrun with demons that hunt in the night. The only protection humans have is some settlement walls painted with special wards that help keep them away.
For an interesting magic system check out The Lightbringer series. It's based off color, with each one having a unique property and psychological effect on the user (e.g. red light creates flammable materials and makes the user get a little enraged). It features technology made from the materials made out of this light and even has a church that governs these magic users.
Will Wight's Cradle, Traveler's Gate, and Elder Empire series nominally take place in the same connected multiverse, each world with its own "magic" system that are all subsumed by an overarching power structure called the Way.
Each series stands alone, with only Cradle exploring the breadth of the interconnected multiverse to any real extent. Elder Empire has some glimpses of it while Traveler's Gate is only connected by oblique references to its world in a Cradle book.
The Last Horizon series could potentially be connected as well, but seems unlikely or at least there's no hints of that yet.
Regardless they're all great.
I would start with Traveler's Gate. Cradle has a similar "rise to power/progression fantasy" theme with a strong lean into the "cultivation" genre and is much longer.
It's funny to me that you would say it's not well written in the same breath that you claimed to enjoy it.
I understand what you mean by that, but I would disagree. The writing completely accomplishes its goal of being enjoyable to read, therefore I would say by definition it is well written.
In retrospect the ending maybe feels a little weak only because the book continues for a while after the true climax, not really tying up loose ends but just showing a relatively relaxed aftermath that contrasts a little oddly with the nonstop building tension of the entire rest of the series.
Since you didn't mention it, you might enjoy "Wheel of Time" by Robert Jordan. Great world building and a unique magic system. Sadly, Jordan passed away before he could finish it, but it was finished by Sanderson. His ending is why I first started reading cosmere.
I've enjoyed The Will Of The Many (J. Islington) between reading/waiting for Stormlight books, it's a bit on the YA side but the world and "magic" system is interesting and the second book should release (I think) in early 2025