Dondoter:
I actually can't consider saving all the data into one file because of the way I want to display the data!
Probably nonsense. But you say nothing about how you actually want to display the data.
The main challenge is the data comes in from all nodes at the same time!!
Also nonsense, the data appears to be received sequentially, the intervals being uncertain, as you have a 100ms delay in the loop but talk about one second. There is no point in guessing about what is really going on, or what you really want, but the most likely time waster you are going to encounter is the opening and closing of sundry SD files, which makes this a rather good time for thinking more sensibly about file management.