I have a dual boot system with Windows and Manjaro.
So… this is what my partitions look like -
where I imagine the ext4 partition is my Manjaro partition.
For the sake of completion, this is my other disk -
To sanely reduce the size of my Windows partition and increase the size of my Linux partition, would it be okay to use Window’s disk management to decrease the size of /dev/nvme0n1p2 and then use gparted from Manjaro to increase the size of /dev/nvme0n1p3?
Yes, it is always best to use the operating system’s own tools for manipulating its partitions and filesystems. So shrinking the Windows partition from within Windows and enlarging the ext4 partition from within GNU/Linux is the best approach.
When you change the / partition of Manjaro - be aware this must be done from a live ISO and this partition must not be mounted. Afterwards, be prepared that you might need to restore the bootloader as you are going to change the start sector of /.
https://wiki.manjaro.org/index.php?title=GRUB/Restore_the_GRUB_Bootloader
P.S.: Except for Data all your partitions are pretty crowded, maybe you can shrink some content, delete some files or think about a larger disk in the near future…
Yikes. Messing around with the bootloader is always scary 
This is what I have been trying to do, but I don’t really have anything I can remove. The problem is that my Windows partitions are broken up with tiny partitions between them - /dev/sda4 and that 1 MB unallocated partition. Otherwise I would be able to store my files on Windows more efficiently and could make my Linux partition larger
Since I am using a laptop, I can’t replace the SSD with a larger one.
You will change your mind as soon as you have succesfully done it once. 
