Do I need complex sentences to achieve Band 7 in Writing or Speaking?
You need a suitable range of grammatical structures used accurately, not a sentence made complicated for its own sake. Simple sentences can communicate important points clearly, and complex sentences help show relationships such as cause, contrast and condition.
Use structures only when the meaning calls for them: “Although public transport is cheaper, some people still prefer to drive” is useful because the contrast is clear. Practise combining ideas naturally, but keep an eye on punctuation, agreement and clarity.
A mixture of accurate simple and complex forms is generally stronger than constant overcomplication.