As someone who doesn't use this mod/play Minecraft extensively, I still want to have support for it in my textures. (currently just working on a personal UI pack, maybe releasing it at a later time.) However, I can't seem to find documentation anywhere about which crosshairs are for what purposes, other than creating a world and trying to figure it out experimentally. Can there be a page on the wiki for this?

Separately, it might be beneficial to separate the various crosshairs into their own images rather than an atlas, just to match what is being moved towards in the base game (but probably not necessary, I guess.)

As someone who doesn't use this mod/play Minecraft extensively, I still want to have support for it in my textures. (currently just working on a personal UI pack, maybe releasing it at a later time.) However, I can't seem to find documentation anywhere about which crosshairs are for what purposes, other than creating a world and trying to figure it out experimentally. Can there be a page on the wiki for this?

As all of the crosshairs are configurable, "what they are for" is entirely dependent on the end user. You can launch the game with yet-another-config-lib installed and go to the settings to see what the default settings are.

Separately, it might be beneficial to separate the various crosshairs into their own images rather than an atlas, just to match what is being moved towards in the base game (but probably not necessary, I guess.)

I don't understand, that is exactly what I did. Do you happen to be looking at a very old version of the mod (for Minecraft before 1.20)?
https://github.com/Crendgrim/DynamicCrosshair/tree/main/src/main/resources/assets/dynamiccrosshair/textures/gui/sprites/crosshair is the folder of all of the individual sprites in the currently supported versions.
ETA: this changed in commit 9420fd0 in October 2023, so I am very confused what you are looking at.