Any tips for trying Mint again without risking my Windows install?

Squashfs errors often mean the installer could not read its files reliably. A new USB is a good start, but also download the Mint image again, check its published checksum, and recreate the installer with a reliable writing tool. If the errors return, try another USB port and, if possible, test the computer's memory too.

Secure Boot usually is not the first thing to change for this particular error. I would leave it alone unless you run into a separate boot or driver issue.

Back up anything you care about before touching partitions. In Windows, use Disk Management to shrink the Windows partition and leave the resulting space unallocated. Then boot the Mint installer and choose the option that installs alongside Windows or uses that free space. Read the disk and partition summary carefully before confirming.

You do not need a second physical drive just to dual boot, but having a backup is worth it. If the installer offers choices you do not understand, stop there rather than guessing.

For a first dual boot, I would keep Windows installed first, make room for Mint, and let Mint set up its boot menu. Do not erase or format the Windows partitions. The installer screens can vary, so check that the proposed changes are limited to the unallocated space you made.

And no, turning off Secure Boot is not a general fix for squashfs read errors. First make sure the installer USB is sound. If you are nervous about changing your only drive, taking a full backup before installing is a better safety step than waiting for another drive.