What would be the quickest way to slightly thicken all the selected edges of this mesh?
I want to turn them into window-frames. Also, is there a way to select all four edges of a square without also selecting the inside face?
Press I twice and move your mouse until you get the thickness you want
Now I’ve got like 40+ thin faces in between what will be my windows. Should I shift select them all one by one separate them from the mesh and then thicken them with a modifier or something? Feel like there must be a quicker way (sorry, I’m a complete noob). Maybe loop cuts were the wrong way to do it to begin with?
I’m not 100% certain what you want to achieve. Do you want to make a door with muntins? Like this (yeah, I know it’s a window, but it has labels for parts)?
Then what Joseph said (“Inset Individual faces”) is a good way to go. Make sure you have scale applied to your object or the inset can be uneven. After the inset you don’t have to select the new little faces individually, you can select several of them at once. One way is to pick Face Select, then ALT-left-click on one of the new diagonal edges in a muntin (that selects the connected ring of faces around the glass), then SHIFT-ALT-left-click on another one to add another ring to the selection, until you have them all selected. And you don’t have to separate them to thicken them, you can just extrude them.
I suggest you take an introductory course to Blender that covers the ground floor of box modeling, such basic things as selections and extrusions, and practice those things. I know that might seem onerous when you just want to get going making your own 3D models rather than whatever the instructor picks to demonstrate on, but it will make the going so much easier in the long run.