Different applications may have their own keyboard shortcuts that are not system-wide, although, importantly, system-wide ones generally override these.

Mint Cinnamon has a Keyboard applet in System Settings that shows the system shortcuts in the aptly named Shortcuts tab. (Xfce is GTK, if not completely GNOME based, so I am assuming it has something similar.)

Unfortunately, at least for this purpose, they're not in one monolithic list, but in a tree view sorted by category.

If I have any sort of recommendation, the number pad is rarely used for shortcuts, especially not with chorded modifiers (e.g. Shift+Ctrl etc.). The closest I'm familiar with are Ctrl-Plus, Ctrl-Minus and Ctrl-0 for changing zoom levels in things like web browsers.

Using lots of modifiers at once is also pretty rare.

Also also, if you have a keyboard with working multimedia keys (or something accessible via an Fn key), these might be detectable with modifiers as well.

Personal examples:

Shift+'e' (a multimedia key, not the letter) = start Firefox with a different profile configuration. Without the Shift is the out-of-the-box default behaviour and the regular Firefox profile. I rarely use this and tend to use launch icons instead, but it's nice to have around.

Super+NumberPad5 = Resize the current window to full height, centred with 4:3 proportions. Another web browser one. I was Internetting on 4:3 monitors for a long time before 16:9 windows were commonplace and browsers still look weird to me at full width.

Shift+Ctrl+Alt+Super+Break = wtf = "Can't hit this by accident" = "Vulcan nerve pinch" = Suspend the computer. I tend to mash Ctrl, Super and Alt at the left of the keyboard where they're all in a line and then hit right Shift and Pause/Break with my right hand.

(I don't have a cat, but I have considered that it's vaguely possible that a keyboard-walking cat could theoretically hit this. Something to think about if you think it's a decent idea but have a cat!)