You can just edit the Magisk root package to ONLY disable the dm-verity flag. It's literally just commenting out a few lines of Magisk scripts, since the functionality is inherently present in Magisk. Here's the link to such a package: Download magisk-onlynoveritypatch.zip Boot into twrp, install from zip, and select this zip.
Good news: trying the following command successfully decrypted my SD card without any errors:
sudo dmsetup create crypt1 --table "0 `sudo blockdev --getsize /dev/mmcblk0p2` crypt aes-cbc-essiv:sha256 [16-BYTE HEX KEY] 0 /dev/mmcblk0p2 0"
A couple of minor issues:
sudo mount -t /dev/mmcblk0p2/crypt1 /media/mint/sd returned the error mount: /media/...