Egr valve not sealing or closing? Cam off one tooth? Really carefully drip timed?

If none of the above I would verify the engine harmonic balancers indicator is dead on by filing the number one cylinder with light oil when approaching the top of the stroke.

Seal a straw into the number one injector opening and advance the engine until the oil level in the straw peaks. This of course should show 0 degrees at the balancers pointer. Just a few considerations I might check. No certainty any of them are the issue.

At least these are the least complex things that occur to me other than air ingress or a very weak fuel supply at idle. Being a slow thinker if anything else bubbles to the surface in the next day or so I will mention it as well. You have to give working mechanics their due. Their instinctual strength is hard to replace. Unfortunatly I am not one.

Before doing a lot of things you have already done. In my case I would have employed the milli volt method in the archives to reduce the shotgun approach. What it would have shown or not shown is almost always signifigant if done with no short cuts. It can avoid the blind leading the blind syndrome when there is no indication of cause.