shrinking partition to move bazzite from sata ssd to nvme

I want to switch how my system is set up and having issues.
Currently I have bazzite on an old sata ssd (1tb) and my old win 11 install on my nvme drive (512gb).
I want to swap the drives to have bazzite on my nvme and windows on my sata ssd.

How do I shrink the system partition so I can clone the ssd to the nvme drive?
I tried it with KDE partition manager and with GParted on a live image but only get a nondescript error.

I found this post about this topic that I have to "do it in / instead /sysroot"
🛟bazzite-helpShrinking the system partition post-install

How would I go about doing that?

edit1: I have backed up all my stuff

edit2: so thanks to chatgpt I now know that I can use sudo btrfs filesystem resize -XXG / to reduce the partition size by xx GB. But this just sets the space as unallocated on the partition.

How do I seperate the unallocated space from the partition? Just using GParted (as mr. gpt suggests) doesn't work
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