Anytime you can go both smaller on the front and larger on the back you are going to gain more low gears. Not knowing the capacity of your derailleurs but knowing that you're unwilling to change your chain yourself tells me that you should put this in the hands of your shop mechanic. You are stuck with the capacity of your front der, but you could possibly go as low as a 26 ring. The rear der could be swapped out for a mtb rear derailleur and then you can go as much as a 34.

Do you use the granny gear much? What type of terrain do you normally ride? A lot of flat with a scattering of hills? For that, I'd prefer the smaller granny and only the 28 cassette. Then you won't lose the close spacing of the cassette for the flatter portions of your rides. On flat terrain, the big jumps between gears on a large cassette drives me bonkers because I always want the gear inbetween.