Any other authors that make hard magic systems as good as Brandon Sanderson?
Okay, I'll start with the one that will devour all your free time and take you forever.
Worm by Wildblow is a web serial about a young girl who develops super powers. She has the "crappy" power of controlling insects…. and does fucking amazing things things with it. (I use the swear there because its important to emphasize) It's dark, internally consistent, and features characters that use their abilities at least somewhat intelligently. The smart characters tend to fare a lot better than the flying bricks that would normally be the main leads. It's also really long. All of Wildblow's stuff is SUPER good, top tier... but too long to be traditionally published. It is absolutely amazing and I wish I had the time to re-read it.
Fans have also put together a fan-made audiobook version. And we had a thread for it here on this forum but I don't think it ever had any traction.
WHen I say it's long, I mean it. It's like 1.7 million words. So roughly the same length as all four Stormlight Archive books, or the first 6 WoT books.
If you end up liking that Wildblow has done other series since and puts out work on a weekly basis, so he'll likely always stay ahead of you. Pact, Pale, Twig, Ward, that are also good… but Worm is the one people know and talk about.
Lightbringer by Brent Weeks is a hard magic system pretty similar to Sanderson's works. 5 volumes, I have only read the first one, I want to get around to the others.
Mother of Learning is about a novice mage who gets stuck in a month-long time loop. The magic system is something of a mix between D&Dish schools of magic and anime-style training exercises. We learn more about the magic system as the protagonist does. Lots of fun twists on the genre in there, too.
Andrew Rowe writes a lot of hard magic systems. Check out Sufficiently Advanced Magic, and if you enjoy that his War of Broken Mirrors series is fun as well.
Rokka: Braves of the Six Flowers by Ishio Yamagata is fantastic if you enjoy fantasy mixed with mysteries. The general premise is six legendary warriors are prophesized to come together and defeat the great evil…but seven members show up. So one of them is a fake. Six books, all released by Yen Press. Another one I need to finish. It also had an anime that adapted the first volume which is how I was introduced to it.
Malazan Book of the Fallen by Steve Erikson is a good series, but VERY hard to read the first time. He throws you into the deep end and does not explain anything and you have to pick up world building and lore from context and a lot of people find that difficult and offputting. As if he started on book 8 instead of book 1. I'm working my way through it but its rough. Supposedly the payoff is fantastic and rereads are super good, I've only heard positives fromtm hose that have finished it… but I know it ends up on a lot of people's DNF pile.
EARTHSEA by Ursula Le Guin's is fantastic. It's more soft magic than hard, but it does have rules and is a fantastic brisk read of a series regardless.
I would recommend Patrick Rothfus's Name of the Wind, but he has a GoT problem where he's two books deep into his trilogy and its been 11 years since the last book and the final volume might never come..