Unity is only testable once the topic sentence makes a real claim — if nobody could disagree with it, every sentence qualifies and the paragraph sprawls. Pick a topic sentence, then run each remaining sentence against it and choose one of the three repairs: cut, subordinate, or split. The revised paragraph rebuilds on the right as you decide.
1 Choose the topic sentence
2 Test every other sentence against it
The question is not “is it related?” Almost everything is related. The question is whether the sentence advances this claim.