Loop for merging multiple BAM files from multiple lanes with different names into related names/ samstat

Hi all,
I’m a beginner in analysis and don’t have any help to check my codes or ask for solutions so I’ll post my questions here.

I have a chip seq data and mapped them with bowtie2. I have 100 bam files with different names. I need to write a loop for samtools to merge them into related file name and then do samstat on each merged file. If I can use GNU parallel to write the loop, it would be awesome.
I have two lanes for each sample and they are single end reads.
For mapping them to genome I used this code:

for i in /~/*.fastq.gz
do
bowtie2 -p 16 -k 1 --fast-local --no-unal --no-mixed -t -x hg19 -U "${i}" |samtools view -o "${i%.fastq.gz}".bam
done

The names are written like this:

First pair:

1. HCT116_Input_S50_L001_R1_001.bam
2. HCT116_Input_S50_L002_R1_001.bam

Another pair:
1.MCF7_K9_I_2_S8_L001_R1_001.bam
2.MCF7_K9_I_2_S8_L001_R1_001.bam

And continues till other pairs and names.
For example, I want them to be HCT116_Input_S50.bam and MCF7_K9_I_2_S8.bam at the end of merging.

I’m using these standalone code:

samtools merge HCT116_input_S50.bam HCT116_Input_S50_L001_R1_001.bam HCT116_Input_S50_L002_R1_001.bam

samstat HCT116_input_S50.bam

Can someone please help me with writing the loops?
Thanks a lot.