From what I can tell from a bit of Googling, the computer was made in or around 2013. If this is correct, then your computer is roughly 5 years old at this point. Depending on history of use, I wouldn’t at all be surprised to see some wear beginning to show on your Dell, namely with thermal management. I see you’ve already blown out the fan and tried things like external fans. That’s a start. You may also need to open the laptop and clean off the old thermal paste from the CPU/GPU/Heatsink and apply a new layer.
Great job on your initial troubleshooting! It sounds like you’ve done a lot of the groundwork already. Do you have a multimeter handy? If so, set your multimeter to DCV and while the adapter is plugged into a wall outlet, place your black probe on the silver tip of the adapter (the part that goes into the laptop) and then place your red probe on the center pin inside the tip. Your multimeter should read 19.5DCV. If this value fluctuates wildly or reports lower than that, your power adapter may be to blame. I have seen many instances where the battery and power adapter were plugged into the laptop and the user assumes that the battery is charging, when in fact, it’s not. After some time of use, the battery drains to the point that the computer powers off.
You may also try updating your computer’s BIOS. This appears to be your computer’s latest BIOS where it addresses and issue with restarting. I checked older versions of your BIOS and while nothing notes about addressing powering off or adjusting power/thermal management, it wouldn’t hurt to update it so see if this takes care of your problem.
So that’s where I would start. Check your power adapter and replace if necessary, update BIOS, and consider disassembling your laptop to clean and apply new thermal paste.