Generally better to use Linux tools for Linux and Windows tools for Windows.
But you probably can delete partitions with Windows if you know how.

You say Windows data is critical, you then do have good backups of your Windows data, just in case.
Its a lot easier to restore a system than it is to attempt to recovery a broken system.

Bit concerned that gparted took so long. It only takes a long time if moving data. But expanding a partition into unallocated space is almost immediate.

I would make sure Windows is backed up and run Smart Status to see if drive has any issues. Windows does not always show issue until it totally crashes.

https://help.ubuntu.com/community/Smartmontools
In Disks alias gnome-disks you can click on icon in upper right corner and see Smart Status
https://www.smartmontools.org/wiki/H...orts_ATA_542.1
If not Ubuntu but other flavor, you can install it.
sudo apt-get update -y
sudo apt-get install -y gnome-disk-utility
Smart Data
https://ubuntuforums.org/showthread....4#post13965854
sudo apt-get install smartmontools