Hand pack the bearings with automotive bearing grease,
Next super clean all the crank and cup threads, then fill the threads on the crank with blue loctite.
Let It setup for a couple of hours before you Install It all.

Several layers of loctite drying out between each application can make a nice thick rubbery thread filler
that can really hold.
Followed by that microscopically too tight thing.

No Idea If that would work but Its what I'd try.