Sorry to hear about your troubles.
Every symptom you have screams power issue to me. Most likely it is a bad PSU or the GPU itself has become faulty. Any PSU will degrade over time delivering less and less power. It isn't always just one day it doesn't work. While I get it worked for you before, if you were right on that threshold between working and not working it would not take much of a degradation to now be an issue. As already suggested the best thing at this point would be to try that card in another computer to see if it is fine there. It's too bad that is not an option. There is a utility that you can do some testing of your PSU, GPU and a lot more and see if it shows anything being an issue. It is called OCCT and you can get it free at ocbase_com.
I would assume you already likely did this but if not check the cleanliness of the system too. A lot of dust in the PSU and GPU can cause big problems.
In looking further into your system and the power it uses. It looks like you really were absolutely under the requirements to begin with. Yes I know others say they were doing it too, I would wonder if many of them ultimately ended up having issues too. I did find several posts of people in forums saying they did and that their power supplies had died or that gpus had failed that they added to those systems. The problem is with delivering too little power it may work and just like the degradation I spoke of that happens with a PSU over time, to little power also causes damage over time. Not delivering enough power can damage capacitors, memory, cpu, gpu, etc.
It is a shame that Dell does all their proprietary stuff and you can't simply use an off the shelf power supply to meet your current needs (pardon the pun, current). You learned a valuable lesson as to why gamers buy systems that use standard parts because you can fix them and upgrade them with ease.
Also lowering the Power Limit for diagnosis as suggest to see if it helps is not a bad idea at all. Also assuming if everything is actually okay still and something else put you over the limit and that is why you are now having issues. Look for anything you may have added new that could be the issue. Another hard drive internal or external USB. Or even other USB devices. Many gaming peripherals with RGB lighting will draw more power through USB than older devices and in you situation anything could put you over the edge.
Run that OCCT test I suggested and see what it shows.
Thanks for the reply. Sadly there is no errors showing yet. I tried the testing, did 2 mins for most of them, 5 mins for the OCCT test. Some didn't give me savable results so I snipped the screen. Here are the results:
Adding to what I said earlier too is that many reviewers point out that on may RX 480 the max tdp is not 150 but in fact 165 so this really again makes a difference when you are walking that fine line between having enough or not enough and damaging expensive equipment over time.
Now about your OCCT results
You need to click
"Power" in the bottom left panel on the main OCCT page under "Test Configuration" :
For instance my result of power with GPU under load:
I did 2 power tests, which are the 2nd and 3rd pictures posted, but for some reason the OCCT app is monitoring the onboard intel graphics and not my dedicated GPU. not sure why. Is there a way to change this?
Edit: When selecting the tests, the RX480 is the GPU selected in the drop down box for testing options.
Done, but the program won't give me the option to save the .png's now? Did I use up all my free ones? I did a 5 min burn un windowed and a 2min burn in fullscreen.
Edit: Snips from mid-test:
Your system seems to be working well. If the PSU had been problematic, you would get an error in seconds.
Uninstall Intel GPU driver and AMD GPU driver with DDU in Windows safe mode. Perform a clean AMD display driver installation. Do not install Intel display driver. Intel driver installation may cause games to fail. If they do not resolve the problem, try RX 480 standard values with WattMan. Or try an older AMD display driver. Your system seems to be very robust.