If you don't use LVM splitting existing ext4 partition might be difficult. To dualboot deepin/windows you need to install deepin manually and leave some space for windows. I don't at what point youre exactly at, but there are plenty tutorials on youtube.

I think I'm going to have to buy a new high-speed USB stix and have a parallel op system on it to mount that and then splice up the EXT4 on my computer, I'm thinking?

I'm a novice in Linux just FYI, from what I'm reading I can create a USB stick with Ventoy on it with many distro loaders and that will give me the option of doing it in a live version virtually whereby I can then cut up and reformat (into whatever format I want) the existing internal drive and then install Windows on it, is that correct? thank you.

Yes, Ventoy works. Any other method to create a bootable live/installation media works as well. So, as long as you can boot a live session, you can then manage partitions. Unlike what happens in Windows you CAN'T make changes to the system partition while running the OS so it must be done in a live session.

Yes, in a live session, you can shrink the current system partition leaving unallocated space that can later be used for dual-booting with Windows (or anything else for that matter). Although you can also format the remaining space as NTFS you don't have to and actually it's recommended NOT to do it. The Windows installer should use the unallocated space to automatically create and format all its required partitions.

Now, there are a few considerations:

1. AVOID Legacy/CSM like the plague unless you're using a machine older then ~2012 with BIOS in which case you must install both OSes in "BIOS" mode. Assuming you correctly installed Deepin in "UEFI mode" - there should be a small FAT32 ESP (EFI System Partition) at the beggining of the drive and this partition is used to save both bootloaders - then make sure you boot the Windows installation media also in "UEFI mode because "how it boots is how it installs" (I always recommend to disable any CSM/Legacy settings in UEFI ("BIOS") - not to be confused with "Legacy USB" that you may or may not need/want and is immaterial for the point I'm conveing here - in order to assure the installation media boots in the desired mode always);
2. Make a note to disable Windows' Fast Startup "feature" and shutdown Windows properly to make it stick, a MUST when dual-booting with any Windows 8.x or newer.
3. After installaling Windows, as usual, you'll notice the system booting Windows directly. This happens because the Windows installer changes the boot order to itself automatically, "for your convenience". You must then change the order back to "Deepin" in UEFI settings > Boot menu (yes, being familiar with your firmware settings is a MUST for what you want to do).

You have the right idea, you cannot shrink a partition when you are in the operating system which uses it. There is a gparted .iso. It is a light-weight version of Debian with gparted, so that you can do what you want with disk partitions.

gparted live

Okay, so you can burn a cd, make a live usb, or drop this .iso in your Ventoy usb and use it. I generally use the third option for VGA display, because the default is hard to see in my high-def monitors. The VGA option is much friendlier on the eyes.

You may want to look up a YouTube video about how to resize and create new partitions with gparted before you do it.

good luck.

edit - If I wasn't clear, there will not be an option in gparted to 'split' a partition. What you have to do is select the partition you want to split and shrink/resize it. Put in the new size you want in mb. For example if it's 300G and you want it to be 150G, you will change 300000mb to 150000mb. After that you will then select the now unallocated 150000 and create a new partition formatted as ntfs and name/label it whatever you want. Perhaps "Windows". When you have it how you want, you click the check mark at the top and gparted runs it.