Issues shrinking the C: drive?

So, I just picked up a laptop running Windows 10 with a Seagate Barracuda, and since it’s all sitting on one giant C: partition, I really want to split it up. My plan is to keep C: at about 200 GB and let the rest—roughly 800 GB—be my D: drive.
I jumped into Disk Management, right? I tried right-clicking on the C: drive, and from what I gather, I'm supposed to type in how much space I want for the new D: partition. But for some reason, it won't let me go past half the size of the C: drive (like, maybe 450 GB max).
Anything higher than that 450 GB limit is totally blocked out—the "Shrink" button is just grayed out and useless to me.
Anyone have a clue why it's being so stubborn and won't let me
? Here's a screenshot of what's happening:

It looks like some Windows system files got stuck right in the middle of the drive, so the OS isn't giving you any more room to breathe. You’re probably gonna need to use a utility to shift things around so you can actually clear out a decent chunk of space.
We're talking about stuff like the boot partition, the page file, or even the Recycle Bin.

Dude, it's a 1TB drive (so about 931GB actual usable space), not 1000MB.

Check out these steps if you're trying to shrink that C: partition. Honestly, Disk Management can be super flaky sometimes and just won't cooperate.
https://superuser.com/questions/1017...s-10-partition
https://www.download3k.com/articles/...-located-00432

If you want to go the 3rd party route:
https://www.easeus.com/partition-man...partition.html

Before you even think about messing with anything, you should probably back up all your data first.

AOMEI Partition Assistant

https://www.resize-c.com/

Oh, and just a little heads-up from my side—please, please make sure you back up all your important files before you start messing around with any partitions, because I've definitely learned the hard way how quickly things can go sideways if you aren't careful!

Best,