You should be able to see the nuclear dust lane reasonably well, but you might have to use averted vision to let your eye see it with its most sensitive retinal area. I have never looked for it in a 12-inch, so I cannot be absolutely certain about this. As for planetary detail, it all depends on what you are looking for. Certainly the bands on Jupiter should show up, and you should be able to see the major divisions in the rings of Saturn. You might need a set of good filters to help remove extraneous light. The bottom line will be the quality of your mirror, and how long you let it thermally equilibrate before you start observing. Pyrex is notorious for changing its figure with temperature.