The extra sentence mentions 5년 (five years) with no context yet for where that number comes from, so it needs to land somewhere that a stretch of time makes sense. Look at what comes right after ㉡: 그동안, "during that time," which only works if a specific span of time was just named. Drop the extra sentence into ㉡, and 그동안 now clearly points back to the 5 years just mentioned. Try ㉠ instead, before the paragraph has even brought up university or graduation, and 졸업한 뒤로는 (since graduating) has nothing to attach to yet. Try ㉢ or ㉣, after 그동안 already had to stand on its own, and you've solved nothing while stranding a sentence about the past in the middle of weekend plans.