You can break it down like this.
- 다만 차이는
- 30 대나 40대의 관점에서 볼 경우 누구나 쉽게 될 수 있는 사람이고(,) 그 속에서의 차이가 나의 현재 삶에 어떤 영향도 주지 않을 것 같은 사람들이지만(,)
- 20대의 관점에서 보면 앞으로의 미래를 어떻게 꾸려나가야겠다는 생각을 가지도록 만드는 사람들이다.
So it is 다만 차이는 [...이지만 ...사람들이다]. It effectively means 다만 차이는 [...이지만 ...사람들이라는 것]이다 (more proper since 차이 = ...것), but they simplified it a bit because it is already a long sentence.
다만 차이는 means "Only, the difference is" or "The only difference is ...". Just before this, the writer said 주변에서 흔히 볼 수 있는 인물들이 나왔다, and now he/she is saying what's different about them, since if all they are is just people you see around you all the time, there wouldn't be much reason to write about them. The writer is now getting into the main points.
Both clause-1 and clause-2 starts with a conditional, like "if you look at them from the viewpoint of someone in their [thirties or forties / twenties]" (볼 경우 is short for 볼 경우에는. 에는 is often dropped_), and continue on with how they might look to you from those perspectives.
It can be roughly translated like this.
- The only difference is that
- viewed from the perspective of someone in their thirties or forties, they are the sort of people you can easily become yourself, so that whatever is different in their lives isn't likely to affect your life of the present in any way,
- but that in the perspective of a twenty something, they do make you think about how you should manage your days going forward.
EDIT: 꾸려나가다 is a compound word of 꾸리다 (1. to pack things (like one's belongings for a move or travel), 2. to manage a living (살림을 꾸리다)) and -아/어 나가다 (to continue something). Here it is used with 미래 (미래를 꾸려나가다) to mean living one's future days with certain objectives in mind, like managing one's life toward one's goal.