If your current soffit lighting is great, I don't see any reason to change to pendants, as I doubt the light quality would change and the look won't be that striking because of the short drop. To improve the quality of lighting at a mirror you would be better off with side-lighting. Overhead light is supposed to be less flattering and casts shadows that make it harder to apply makeup, etc.

Here is one in chrome that extends 9 inches. http://www.destinationlighting.com/item/capital-lighting-capital-vanities-chrome-bathroom-light/P1452623 We went with this one in brushed nickel that extends 9.5" and have it with the lights down. http://www.destinationlighting.com/item/feiss-lighting-hamlet-satin-nickel-bathroom-light/P1523323 There are some out there and I am happy to help if you give me details on what you are looking for. I went through this search earlier this year and spent weeks looking. We have a surface mounted medicine cabinet by Bertch that sticks out only about 4 inches. There might be other medicine cabinets. I went through that search too.

18" W LED bathroom light for above medicine cabinet

I have done a box before. In my case, the light fixture ended up on the slanting part of the ceiling and the fixture could not accommodate the slant. So I had my carpenter build a box to match the slant and make a level place for the light to attach. So you can have a box made for behind the light fixture. Others have posted here, asking for help finding a light that protrudes enough, and I don’t think any have been found. Can lights have their own issues above a vanity. They usually make shadows on the face of the person shaving or doing make-up. Light behind you may help. These ceiling lights are very easy to install and they send out the light 360 degrees around, plus, their color can be adjusted. They might help in your situation. One over the shower is nice, too. I found them when doing my bathroom and loved that they need no electrical box behind them and take up only the depth of the ceiling drywall. My builder had used them before and also loved them. You don’t have to worry about ceiling beams as long as the electrician can get the wire to them. https://www.build.com/dals-lighting-5006-cc/s1763172?uid=4163442