I loaded a bunch of flat base, 1r 9mm 150 grain boolits as an experiment to see if I could get lower recoil, better accuracy from my 9mms. The most accurate 38 special load I ever worked with was 200 grain SWC and 2400 powder. Had a recipe that made major and was not +p. I was looking for the same thing in 9mm...more bearing surface, more weight, better penetration for a "woods gun" and of course it had to feed.
The taper issue is a big deal. With soft lead i believe you can use the Lee factory crimp die and kind of "iron out " any bulge you might get. Not good, but if the alloy is soft it will "slug up" under firing pressure. Mine did. No leading and mild recoil . I only experimented with fast powders. I was looking for low recoil, quick recovery from recoil. It is also nice to shoot a 9 and not have the ear splitting sonic crack. I have read of British loads for suppressed STENs that used 160- 170 grain boolits but I can't remember where and don't remember many details.
9mm cartridge development has indeed gone the wrong way. Faster, lighter bullets make pretty flowers in gelatin but big heavy blunt boolits make gaping bleeding holes all the way through critters and tend to shoot without the snap and crack of the lighter faster loads. I think 9mm will run out of room before the recoil toleration level and recoil recovery level of most folks is reached. There really is something Special about the 38 special. A 38 Super is a much better platform. Probably a bout perfect case size.