If you don't sand it, and possibly take the stickers off the paint will not stay on the bike. Even if you do everything right, and use primer, the paint will be much easier to chip and scratch than the original factory baked on paint.

If the stickers are easy to get off, I'll certainly remove them.
And I would lightly sand the paint, of course, just not strip the paint off entirely.

If it comes off, I don't really mind, as I can paint some more back on at a later time.
I'm not looking for a nice finish, so any small bumps or ridges won't bother me.

OK now I get it. I have two self painted bikes myself, I found that the color black does not show touch up paint or scratches as much as other colors. Of course your plan does not really need to work that way.
It's a good plan, I would love to see a photo when you're done.

These two are spray painted with a can, I take these when I know it's going to rain or they will be scratched, like on a ferry boat etc.

Sold on Bianchi eh? At least they are consistent...

The current paint is a somewhat glossy black.
So a small scratch shouldn't be too visible against a matte grey or olive paint.