As the owner of a 1980 240d manual shift myself I suggest that you turn up your turbocharger a little...since you are at such an extreme elevation.. it sure helped on mine....
LOL....

The first and cheapest things to do IMO is to do the Diesel Purge by the book, check your timing chain stretch and set the IP. I do not know about the bubble method .... I have not seen that in the FSM... I think that was gotten from the guy in Waco....

I don't know what you mean by " no visible chain stretch".... the FSM specifically says " if you have a performance complaint" and YOU DO,.... then the stretch must be checked using the 2mm movement of the number one intake valve method.....

I bought my car cheap because number one cylinder read very low compression and it was using a quart of oil per 300 miles... but it is very perky and always gets 24 mpg...it had some oil leaks and I know it needs valve seals....