How to move files to different folders based on their prefix?

• I have thousands of files in my home folder that look like this:

> ls 

6_2_S28_R1_001.fastq.gz  
19_1_S160_R1_001.fastq.gz
25_3_S114_R1_001.fastq.gz

• Here, as an example, I break down the name of the file 6_2_S28_R1_001.fastq.gz;
  6_ = day6
  2_ = replicate 2
  S28_R1_001.fastq.gz = extension
  

• In the same directory I have the folders

rep1
rep2
rep3

My goal

I am trying to move each file to a different folder based on the prefix, that is related to the replicate

6_2_S28_R1_001.fastq.gz,   #this file needs to be moved to rep2
19_1_S160_R1_001.fastq.gz, #this file needs to be moved to rep1
25_3_S114_R1_001.fastq.gz, #this file needs to go to rep3 folder

I have tried an unsuccessful for loop. What can I try next?

#! /bin/bash
for f in *_*_*_*_*.gz; do
    i="${f#*_}"; i="${i%%_*}"
    d="rep$i"
    echo "Move '$f' to '$d'"
    mkdir -p "$d" \
    && mv "$f" "$d"
done