Linux

i3 uses Super key combinations regularly, fwiw

Hyper + [whatever]

If you are ready to use a bit of remapping (try keyd, it's awesome), then you can remap a key you rarely use to Hyper to get that as a direct modifier.

You can also assign some key to activate a layer when held, but still return it's regular output when tapped. So e.g. holding A gives you a new layer, press-and-release just prints A.

In the new layer, you can then set, say, f1 = f13... f12 = f24 and, say, esc = katakana etc., thereby getting access to a lot of keys not present on standard western keyboards, therefore not used as standard shortcuts. These you can then use directly, or in combination with standard modifiers like Ctrl, Shift...

Alt/Ctrl+___ are usually used by applications and shortcuts containing Super+___ are usually "reserved" (it's no rule or anything but basically no application uses Super) for the DE. That's why Super is probably the best mod key for shortcuts. You can ofc use Alt+___ as well, in that case your DE/WM will just take preference over the application and the shortcut will be handled by the DE/WM instead of the application

there a way to know what your systems current shortcuts are

Not really, besides just reading the manual. I think this is a problem for the Linux desktop actually. I would love a standard way (dbus API?) for the DE and various apps to declare their key shortcuts, and then I could view them in a pop up when I'm using the app.

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!)

shortcuts can be customized in almost any DE.

like xfce.

and websites use browser shortcuts, which again can be customized.

i dont know if i understand your question well enough. but i would guess any 3 key combination of non standard shortcut combo (~~cmd~~ctrl+alt) will lead to unique shortcuts.

edit: ctrl not cmd. Im dumb.

Printscreen + Screenlock + Pause ?

I use shift+super or ctrl+super as a modifier for shortcuts. Most of my things ignore when the super is also pressed. I also removed all of the shortcuts from XFCE's keyboard settings so only mine are there.

EDIT: I also want to say I use a Razer Naga X mouse, and have access to a full numpad with my thumb.

I don't know much about xfce, but there should be config file somewhere with all shortcuts, at least that's how it works in hyprland. And btw, I use super (windows) key for shortcuts related to my wayland compositor (like switch workspace, swap windows, close window, and so on). I've never run into any conflicts with this.