So you're expecting your lawyer to be your dentist also ?
For sure - generalists .. it is when you can do lot of things but none of them are mastered.
You can start being a generalist but if you want to master something you need to narrow down your focus.
Imagine you have 100% of your skills / attention / dedication - you would invest 25% in concepting, another 25 in designing, then another 25 in visualising, and to finish the circle another 25% in presenting (selling) .. that is great if you're going to be cheap and aim for a low quality ...
If you dedicate yourself to design you .. for sure can think about how it wold be visualised later .. and about possible light solutions and materials and etc, but without mastering visualising on the same level as design - you will always fail to deliver the top quality images.
Visualising means studying the light, the photography, the composition, the colour theory, the light / colour mixing ... in fact studying and mastering those might take you much longer than studying design ... you can study both of course ... sacrificing quality of both.