You seem to have tried almost everything except -
Have you a spare PSU which you can try in substitution?
PSU's are quite sophisticated with circuits that keep the supply voltages within quite strict limits. However if a component fails it can lead to loss of useful current as soon as any load is placed on it.
One other thought. When you tried disconnecting "everything" did that include the HD?
If the HD has a motor fault it could drag the supply voltage down with it.
Booting without an HD will give a "drive not found", or boot failure or something like, but if you can go that far showing on the monitor it will absolve the PSU.