Tanya-
Altering the shoulder, neck, and armhole area is challenging and not always possible without essentially remaking the bodice. Slimming waist and hips is typically quite easy.
If sheath dresses are going to be a large part of your wardrobe it might be worth looking for a seamstress to make them for you. Once a basic sheath is fitted changing details is quite easy.
Hope this helps.

Tanya, I know I don't post pictures often, but for a while I've thought that you and I have very similar body shapes. We're about the same height, but I am a few sizes bigger. Usually I can take a 6 in a sleeveless dress but have to size up to an 8 in one with sleeves.

Of the sleeved dresses I have now, two are a stretchy bodycon jersey material and no alterations were needed. One Vince Camuto and one Eliza J fit perfectly after waist and hip alterations but are still shorter than I'd prefer. I have the best luck with getting the length I like from Boden and sometimes order a tall from them. The shoulders usually fit right in an 8 and the arms are long enough, but I do have to take in waist and sometimes hips.

Maybe try these dresses, Tanya. I forgot all about good old KM: