3 years of XPS 9560 slowing to a halt, Integrated GPU at 100%

I beg the help of you wise people. I have only been able to use my dell XPS 9560 for basic tasks for three years.

Problems experienced:

After around 1 minute of starting to game (literally any game that requires any effort) or sometimes even when I have multiple tabs open and am trying to watch more than one video, the following occurs:

- 80-100% load of the Integrated HD GPU 630. Processes responsible appear to be "Desktop Windows Manager, Client Server Runtime Process"

- Laptop becomes unusable for upwards of 20 minutes

- I also see that every time I move my razor gaming mouse quickly across the desktop, the above processes seem to react and jump up another 10-20%.

Solutions attempted (all with no effect)

- Re-booting windows, doing a clean install

- Cleaning my internal cooling fans on the laptop

- Using NVIDIA control panel to ensure that the GTX 1050 was employed by most apps.

- Tried to get all updates for drivers and firmware etc as I can (using dell support assistant and online)

- Run multiple virus scans (e.g. with bitdefender)

- Reducing polling rate of my gaming mouse (made the quick mouse movements described above only load the GPU to 10% instead of 30% but had no effect on the overall problem).

- Playing around with desktop scaling as apparently that may have had some effect.

Things that do help:

- Turning off my computer for a long time, after which it takes a little longer to stop working

- Used to be that disconnecting from my external monitor (Dell Ultrawide U3417W) helped but that is no longer the case.

My Device Specifications:

Laptop: XPS 15 9560; i7-7700HQ CPU@2.80GHZ; GTX 1050; 16GB RAM

External Monitor: Dell Ultra-wide U3417W

Please, if you could offer me any help that would be amazing. Please also bear in mind that I'm not very good with computers, so if you suggest something could you avoid acronyms and treat me like someone who knows nothing. Thanks!
I don't know if you fixed you laptop as yet or not. I have a Dell XPS 15 9550 with touch screen 16gb RAM 1tb NVME HD and it performs very well. Yours should perform even better.
You said you did a fresh install of Windows 10 (I hope it was the 64 bit version).
You also said you installed the Dell drivers and some Dell software. Checked for virus etc.
I would do another fresh install Windows 10 64 bit but make sure you have the Windows key before you do a reinstall. If you don't know what it is download and run a free program caller Belarc Advisor. Its a free program but you now have to sign up for it. This program provides the full specs of your machine and software licenses.
Don't put any of the Dell BLOATWARE on your machine. Just let Windows 10 pick up all the drivers. It should go straight on. You may need the wireless network driver before you start, so get that and have it ready.
Do all the Windows 10 updates. It may take a couple of days the get to the latest version of 2004.
You could also put on a couple of free programs from Iobit. Advanced System Care, Driver Booster (which updates to the latest drivers) and Iobit Uninstaller. The Uninstaller program is very good and removes all (most) the rubbish the Windows uninstall process leaves behind.
You can uninstall some of these programs after you have run them.
Ccleaner is another good (free) program and does similar things to Advanced System Care. I run both, however, I don't play games.
My machine is lighting fast and yours should be even faster.
Overheating may be your problem. You could purchase a set of torque screw drivers (t5) and open up your laptop carefully remove the two fans and heatsink and clean out any lint and fluff. If you remove the heatsink, you should apply new heat conducting compound. I use MX4 which is available from eBay. It's not the cheapest but it's the best.
Don't ever use your laptop on a soft surface like a bed. Laptops draw in air from the bottom and expel it from the rear. It also fills up the fans and heatsinks with fluff and lint. If you must use it on a soft surface like a bed, place it on a stand a few inches off the surface. I don't like those so-called laptop coolers which run off usb power. The suck air out the bottom and fight against the laptop fans attempting to draw in air from the base.
I know this is old, but I'm having almost the same experience from my Asus FX505GT all of a sudden. Used for 1 years with no problems, now Windows Desktop Manager (dwm.exe) is spiking my integrated GPU to 100% frequently with simple tasks (scrolling down website). Much worse when connected to external monitor (unusable). Better when not connected to external. I reset my computer to factory install, tried pausing windows updates, and problem occurs again - but, windows keeps managing to install a few latest updates (including 20h2). Any update on this issue? Seems software, but could there be a hardware cause to this? So frustrating.