How can I remap my keyboard to create a lefthand numpad?

I am into custom keyboards running QMK and such which allows to redefine every key and allows advanced behaviours such as layers. I recentley came across Kmonad which is a software tool which allows most of the same features. I haven't tried it myself, but this should allow you to hold down or toggle a key to activate a layer on which you can map numbers to the left side of the keyboard.

Here is a link to the github; https://github.com/david-janssen/kmonad

I think Kmonad is also included in a lot of recent distros so it might already be available in APT.

You have already mentioned that you can edit /usr/share/X11/xkb/symbols/, I think that this is the most portable option and the one that has worked for me in many linux distributions without needing to install anything. I found some issues in Android Studio but not with the numpad. Most software including browsers and DuckDuckGo work as intended.

If you are willing to give another try, I use Alt Gr and the right hand for the numpad. I use the left hand for other keys like Home, End, del... and these are even more useful than the numpad for me.

Create a new language making a file like /usr/share/X11/xkb/symbols/aa based on the language you normally use.

in the file /usr/share/X11/xkb/rules/evdev.xml

sudo gedit /usr/share/X11/xkb/rules/evdev.xml

before </layoutlist> add this text

<layout>
  <configItem>
    <name>aa</name>
    <shortDescription>Prototype700</shortDescription>
    <description>Prototype700</description>
    <languageList>
      <iso639Id>eng</iso639Id>
    </languageList>
  </configItem>
  <variantList/>

And choose that language as your default one. Below the one I use, based on xkb/symbols/gb

// based on a keyboard map from an 'xkb/symbols/gb' file

default  partial alphanumeric_keys
xkb_symbols "Prototype700" {

    // Describes the differences between a very simple en_US
    // keyboard and a very simple U.K. keyboard layout defined by
    // Prototype700

    include "latin"

    name[Group1]="English (UK)";

    key <AE02>  { [         2,   quotedbl,  twosuperior,    oneeighth ] };
    key <AE03>  { [         3,   sterling, threesuperior,    sterling ] };
    key <AE04>  { [         4,     dollar,     EuroSign,   onequarter ] };

    key <AC11>  { [apostrophe,         at, dead_circumflex, dead_caron] };
    key <TLDE>  { [     grave,    notsign,          bar,          bar ] };

    key <BKSL>  { [numbersign, asciitilde,   dead_grave,   dead_breve ] };
    key <LSGT>  { [ backslash,        bar,          bar,    brokenbar ] };

    key <AD01> {    [     q,    Q,    Escape,       Adiaeresis ]    };
    key <AD02> {    [     w,    W,    Delete,            Aring ]    };
    key <AD03> {    [     e,    E,    Up,           Eacute ]    };
    key <AD04> {    [     r,    R,    Prior,       registered ] };
    key <AD05> {    [     t,    T,    Prior,            THORN ] };
    key <AD06> {    [     y,    Y       ]   };
    key <AD07> {    [     u,    U,    7,           Uacute ] };
    key <AD08> {    [     i,    I,    8,           Iacute ] };
    key <AD09> {    [     o,    O,    9,           Oacute ] };
    key <AD10> {    [     p,    P       ]   };
    key <AD11> {    [ bracketleft,  braceleft   ]   };
    key <AD12> {    [ bracketright, braceright  ]   };

    key <AC01> {    [     a,    A,    Home,           Aacute ]  };
    key <AC02> {    [     s,    S,    Left,          section ]  };
    key <AC03> {    [     d,    D,    Down,              ETH ]  };
    key <AC04> {    [     f,    F,    Right,                F ] };
    key <AC05> {    [     g,    G,    End,                G ]   };
    key <AC06> {    [     h,    H       ]   };
    key <AC07> {    [     j,    J,    4,                J ] };
    key <AC08> {    [     k,    K,    5,               OE ] };
    key <AC09> {    [     l,    L,    6,         Ooblique ] };
    key <AC10> {    [ semicolon,    colon,     ntilde,           Ntilde ]   };
    key <AC11> {    [ apostrophe,   quotedbl,  dead_acute, dead_diaeresis   ]   };

    key <AB01> {    [     z,    Z,    BackSpace,               AE ] };
    key <AB02> {    [     x,    X,    Insert,                X ]    };
    key <AB03> {    [     c,    C       ]   };
    key <AB04> {    [     v,    V,    Next,                V ]  };
    key <AB05> {    [     b,    B,    Next,                B ]  };
    key <AB06> {    [     n,    N,    0,           Ntilde ] };
    key <AB07> {    [     m,    M,    1,               mu ] };
    key <AB08> {    [     comma,    less, 2,           less ]   };
    key <AB09> {    [    period,    greater, 3,     greater ]   };
    key <AB10> {    [     slash,    question    ]   };

    include "level3(ralt_switch_multikey)"
};

If you use Windows machines, Autohotkey is your friend. You probably know that, since you are familiar with Autokey.