integer'image(some_int) results in "Expression is not constant" error

I don't see how else I would do it. The ROM isn't really a rom anymore. I converted it into the code below.

entity char_display is
    generic(
        text_length: integer := 1;
        addrWidth: integer := 11;
        dataWidth: integer := 8
    );
    port(
        text_to_be_displayed : in string(1 to text_length) := (others => NUL);
        origin_x : in integer;
        origin_y : in integer;
        current_x : in integer;
        current_y : in integer;
        this_px_is_text : out boolean;
        this_px_is_text_bg : out boolean
    );
end char_display;
architecture Behavioral of char_display is
   
    type rom_type is array (0 to 2**addrWidth-1) of std_logic_vector(dataWidth-1 downto 0);
    -- ROM definition
    signal ROM: rom_type := (   -- 2^11-by-8
        -- NUL: code x00
        "00000000", -- 0
        "00000000", -- 1
        "00000000", -- 2
        "00000000", -- 3
        "00000000", -- 4
        "00000000", -- 5
        "00000000", -- 6
        "00000000", -- 7
        "00000000", -- 8
        "00000000", -- 9
        "00000000", -- a
        "00000000", -- b
        "00000000", -- c
        "00000000", -- d
        "00000000", -- e
        "00000000", -- f
        -- SOH: code x01
        "00000000", -- 0
        "00000000", -- 1
        "01111110", -- 2  ******
        "10000001", -- 3 *      *
        "10100101", -- 4 * *  * *
        "10000001", -- 5 *      *
        "10000001", -- 6 *      *
        "10111101", -- 7 * **** *
        "10011001", -- 8 *  **  *
        "10000001", -- 9 *      *
        "10000001", -- a *      *
        "01111110", -- b  ******
        "00000000", -- c
        "00000000", -- d
        "00000000", -- e
        "00000000", -- f
        -- STX: code x02
        "00000000", -- 0
        "00000000", -- 1
        "01111110", -- 2  ******
        "11111111", -- 3 ********
        "11011011", -- 4 ** ** **
        "11111111", -- 5 ********
        "11111111", -- 6 ********
        "11000011", -- 7 **    **
        "11100111", -- 8 ***  ***
        "11111111", -- 9 ********
        "11111111", -- a ********
        "01111110", -- b  ******
        "00000000", -- c
        "00000000", -- d
        "00000000", -- e
        "00000000", -- f
                .
                .
                .
    );
begin
    process (text_to_be_displayed, origin_x, origin_y, current_x, current_y)
        (variables...)
    begin
        (combinational logic...)
    end process;    
end Behavioral;

It was originally this: https://github.com/madlittlemods/fp-v-ga-text

but in my opinion it was super over-engineered. I reduced that mess down to the one entity shown above which is working great.

My question is, if the way I'm doing it is so backwards, how would you describe a process that is equally code-minimal, easy to read, and efficient?