Motherboard can be tested with different RAM+CPU+FAN and PowerSupply (which you know are working).
CMOS battery normally lasts 2-3 years (recommended replacement). Re-seat onboard jumpers. Clean dust from MB surfaces with gentle brush (because it may act as electrical conductor).
Try to disconnect all devices again (removing all internal modules, including M.2 storage and M.2 WiFi), but this time don't plug everything back (very careful with drives if you have RAID array, you may lose data, and if clearing BIOS/CMOS - ensure you remember all settings) plug only essentials - power to motherboard, single memory stick, no keyboard/mouse or any other externals, (just to eliminate possibility what one of additional devices is causing this), internal speaker (in case if it tries to beep you fault codes). do you have spare cooler? Or Fan for cooler? (those have bearings which can last 3 years of cheaper models) Re-plug power button? 7 years is enough to break mechanical elements, as well as to get thermal compound under CPU dry (normal longevity is 3 years, some higher tier thermal paste lasts up to 5 years), that's not only about CPU but affects chipset and (if installed) VR heatsink. Some type of capacitors over 7 years do degrade drastically, as well as elements of Voltage Regulator.
If nothing works (incl. reseating CPU) then one of remaining elements (MB or CPU) is most likely toasted (after 7 years of good service).
-------------- I am not a Dell employee or affiliated in any other way, just professional IT contractor and life-long technology enthusiast based in London/UK, so do not mind sharing thoughts and ideas with community.