Okay. Let me start here: you have an iPhone, but no mention of any other Mac device - computer, iPad, iPod touch? This is important to know because another Mac device, if linked to your iCloud account, could explain some of your troubles. You do have a Windows computer however.
And you do have an iCloud account with 200GB storage = £2.49/month or equivalent.
You said: "I have an iPhone 11 (256GB) that is filled up with pictures to about 250GB." That leaves very little room for any kind of manoeuvre.
I've read your subsequent posts and won't bore you with quotations.
This is what I suggest. On your iPhone, go into Settings > Apple ID > iCloud > Photos and turn OFF iCloud Photos and My Photo Stream.
And as a confirmation of this, go into Settings > Photos and turn off iCloud Photos and My Photo Stream if not already turned off by above actions.
This disengages you from iCloud. Your iPhone is now stand-alone.
Next: sign in to iCloud on your Windows PC as you did previously. Seek out the 11K of photographs and download them to your Windows PC or to an External Hard Drive (EHD) - I apologise for not knowing much about Windows, but as you've downloaded the pics previously - in 1000 picture blocks/100GB, do that again - unless you are certain that those 11K photographs are definitely on your EHD. No need to repeat that if they are on your EHD for sure.
Then we can proceed.
Your iPhone is isolated from iCloud (as above). We now have to delete all your photographs from the iPhone. Theoretically, if you do NOT have any other device linked to your iCloud account, you could delete all 11K pics from iCloud and then try to to sync iPhone with iCloud; but I don't recommend that route. Even if that worked, your iPhone with little to no extra storage space, would take ages and ages to delete those pictures.
Here are better options:
Firstly, you could use your Windows PC to do the job by connecting your iPhone to the PC via USB. And then:
Once connected, navigate to your iPhone's DCIM photo folder within Windows Explorer and select your photos you wish to delete, or
Ctrl + A to select all. Now delete them by
right-clicking or Shift + Del to permanently delete them from your iPhone. Takes a long time but much, much quicker than the sync method.
This is taken from:
Want to quickly delete photos from your iPhone? Here's how!
That should work - but if not:
Another option - not so wild as you might think - find anyone with a Mac computer. Permissions and ownership doesn't matter in this case. Connect your iPhone to the Mac via USB. Open Finder > Applications > Image Capture app.
You will see your iPhone name on the left. Click on that. On your right, all 11K photographs should appear. Click on the first one to highlight it. Then use Command plus A to select them all. Then Right Click on a photo. You are given two choices: Import all or Delete All.
Choose Delete. It will take a couple of hors at least.
That's the best I can come up with at short notice. Please, please do let us now how you get on.
Good luck.
Ian