compressing and decompressing dd image - zstd instead of gzip

Using dd like that (without any options) will make your life miserable. Just cut it out entirely. Or at the very least increase its block size and tell it not to object to short reads.

1. Without dd, first run sudo -s to get a root shell:

   gzip </dev/sda2 >/media/mint/Data/_Fsarchiver/MintV1.img.gz
   gunzip </media/mint/Data/_Fsarchiver/MintV1.img.gz >/dev/sda2
   

   Your zstd commands look entirely plausible, but just omit dd and read/write the device directly as root. (My version doesn't understand your T6 so I've omitted that here.)

   zstd -16v </dev/sda2 >/media/mint/Data/_Fsarchiver/MintV1.zst    
   zstdcat -v /media/mint/Data/_Fsarchiver/MintV1.zst >/dev/sda2
   

2. With dd, either prefix the dd with sudo or use sudo -s to get a root shell:

   dd bs=1M iflag=fullblock if=/dev/sda2 status=progress | gzip >/media/mint/Data/_Fsarchiver/MintV1.img.gz
   gzcat /media/mint/Data/_Fsarchiver/MintV1.img.gz | dd bs=1M iflag=fullblock of=/dev/sda2 status=progress
   
   dd bs=1M iflag=fullblock if=/dev/sda2 status=progress | zstd -16v >/media/mint/Data/_Fsarchiver/MintV1.img.zst
   zstdcat /media/mint/Data/_Fsarchiver/MintV1.img.zst | dd bs=1M iflag=fullblock of=/dev/sda2 status=progress
   

3. With pv instead of dd. Either use sudo pv or sudo -s beforehand to get a root shell:

   pv </dev/sda2 | gzip >/media/mint/Data/_Fsarchiver/MintV1.img.gz
   gzcat /media/mint/Data/_Fsarchiver/MintV1.img.gz | pv >/dev/sda2
   
   pv </dev/sda2 | zstd -16 >/media/mint/Data/_Fsarchiver/MintV1.img.zst
   zstdzcat /media/mint/Data/_Fsarchiver/MintV1.img.zst | pv >/dev/sda2
   

   Also see Syntax When Combining dd and pv

zstd supports same commands and pipe capabilities as gzip, so if the set of commands works with gzip, it will work with zstd too.

As a minor comment, note that several command flags on the decompression side are redundant: zstd -dvc would be enough and work the same, since -f and -T6 are not useful for this scenario (though thankfully they also don't hurt).