Help Wrap My Mind Around G-Shift & How To Setup Mouse As Modifier

My mouse is a Logitech G502 and I am trying to set it up so it can use the buttons as modifiers in the game. I ideally want it setup so it’s a fairly universal profile for the majority of MMO games just for consistency sake.
For example: SpellX = Shift+1
Hold button on mouse = Shift + Press 1 on keyboard = SpellX

Using the keyboard for spells It is simply uncomfortable, and I can’t reliably hit stuff like Shift+1, Ctrl+1, etc. (Also Alt as a modifier is dangerous because Alt+4 risk haha)

Now, I am having trouble understanding G-Shift and how it translates to usage in games.
First let me clarify my current binds on the mouse.

Under Scroll Lock: Profile Switch
Bottom LMB: DPI Cycle
Upper LMB: n/a
Side Front: n/a
Side Back: n/a
Side Front Square: G-Shift

(Technically Upper LMB, Side Fr, Side Bck are bound to numpad numbers but ignore this for now, I clarify why later).

Now, if I simply go into WoW, and try to edit a keybind by holding G-Shift and selecting 1, nothing happens. It just shows as ‘1’.
Now, this makes sense if I understand G-Shift correctly, it modifies the inputs on the mouse. Because in G-Hub, you can select different key binds for normal, and key binds for G-Shift mode. So for example, Side Front can be ‘1’ and when G-Shift is held it can become ‘2’.

But here is where I am confused. I refuse to believe that while holding the G-Shift (even if assigned to any of the other buttons) can allow you to also reliably press a 2nd button on the mouse (that isn’t LMB/RMB).

What I want G-Shift to do is when I hold it, it acts as a modifier like Shift.
Now, I know I can simply bind that button to be Shift, but now I’m confused as to the entire point of having the G-Shift button?
Is the G-Shift button maybe intended to be for their keyboard lineup that also has G-Shift, and you press it on the mouse instead of the keyboard for easy access? That would make sense to me. Unless I’m just being stupid here?

Now my final question being, how should I set this damn thing up?

Currently, I was thinking just set Side Fr = ‘Shift’, and Side Bck = ‘Ctrl’, and if the G-Shift button (is useless?) then change it to ‘R-Ctrl’ or something (Again Alt seems dangerous lol).
Currently I have them all bound to Numpad numbers actually, but it didn’t work in game. WoW didn’t recognize them as modifiers.

I have little MMO experience in general, and even less when it comes to macros and modifiers. I’ve had the mouse for a while and used the side buttons for single things like crouch, walk, voice ptt, or a single spell. I figured I’d optimize my setup now that I am finding myself playing more MMOs, because Shift, Alt, Ctrl, while hitting keys on the keyboard is extremely uncomfortable and just not fun.
Also on a side note, I absolutely love how the mouse has on-board memory.