How can I switch from Windows 11 to Linux Mint without losing my files?

Follow the 3-2-1 backup rule whenever possible: keep three copies of important data, on at least two different types of storage, with one copy stored somewhere else. Since you currently have no backup and both internal drives are almost full, I would avoid deleting the second Windows installation or resizing anything until you have copied the astrophotography files somewhere safe. After that, you can install Mint into newly freed space and remove the unused Windows partitions carefully.

If you can create enough unallocated space by shrinking a partition, Mint can be installed there without deleting your existing data. Do the shrinking from Windows first, after removing unnecessary files and defragmenting if appropriate. Even then, keep in mind that partition changes are not risk-free. Linux can read and write NTFS, but using an NTFS partition as the main location for Linux applications or games is generally less ideal than using a Linux filesystem such as ext4.

The safest answer is to make a backup before changing partitions. Resizing a nearly full drive can work, but it always carries some risk, and without another copy your files could be lost if something goes wrong. Back up the personal files you care about first; there’s no need to preserve the Windows installation itself, and Steam games can usually be downloaded again. Cloud storage may help for smaller amounts, but 600–800 GB of free storage is difficult to find, so borrowing or buying an inexpensive drive would be much safer.