Hey!
Band 6 criteria for a Business Studies essay requires a "sustained, logical and cohesive response". Showing links between topics definitely shows the marker the depth of your knowledge, but you need to make sure you still answer the question. Its easiest to make links to the strategic roles of the topics. E.g. in a marketing essay you propose a price skimming approach and link it to the strategic role of operations goods/service differentiation.
I got 20/20 for one of my essays and I barely mentioned other topics (I had only learn operations and marketing at the time). I think it is more important to provide links within the topic e.g. talk about how two strategies work together e.g. branding and relationship marketing. Mention how they respond to influences. You also need to show you case study knowledge. I usually use 3 case studies per essay.
But with all things considered I think the most important thing to do is answer the question. Essay questions only ever ask you to respond to one topic, so you should address this first (also you don't want to be mentioning another topic so often that your marker things you've gotten confused about what you are trying to say)
Hope this helps!