Why can't I get the letter F to be aligned nicely

What you are seeing are the sidebearings of the character.

Try this code (needs lualatex) for turning the switch dropsidebearings on and off. Courtesy of Marcel Krüger.

See Dot won't properly align with text and links in it.

    % !TeX TS-program = lualatex  

documentclass[12pt, a4paper]{article}

RequirePackage{showframe} % margin line
renewcommandShowFrameLinethickness{0.1pt}
renewcommandShowFrameColor{color{red}}

RequirePackage{luacode}
newcountdropsidebearings

begin{luacode*}
    
    ------------------------drop_sidebearing------------------------
    --https://tex.stackexchange.com/questions/427068/sidebearings-and-precision-left-right-alignment?noredirect=1&lq=1
    --In LuaTeX, you can use the post_linebreak_filter to intercept the broken lines and
    -- add some offsets. The sidebearings can be extracted from rawdata saved by luaotfload.
    -- After adding the offsets, the hboxes have to be repacked to determine the new glue settings.
    
    ---updated for 1.09
    --https://tex.stackexchange.com/questions/470276/perfect-alignment-luatex-and-sidebearings-part-ii
    ------------------------drop_sidebearing------------------------
    
    
    local function drop_sidebearing(head, groupcode)
    
    if tex.count['dropsidebearings'] == 0 then
    return true
    end
    for n in node.traverse_id(node.id'hlist', head) do
    local char = node.has_glyph(n.head)
    if char then
    local f = font.getfont(char.font)
    if f.shared then
    local kern = node.new(node.id'kern')
    kern.kern = - f.shared.rawdata.descriptions[char.char].boundingbox[1]*f.size/1000
    n.head = node.insert_before(n.head, char, kern)
    end
    end
    for ch in node.traverse_id(node.id'glyph', n.head) do
    char = ch
    end
    if char then
    local f = font.getfont(char.font)
    if f.shared then
    local desc = f.shared.rawdata.descriptions[char.char]
    local kern = node.new(node.id'kern')
    kern.kern = - (desc.width-desc.boundingbox[3])*f.size/1000
    node.insert_after(n.head, char, kern)
    end
    end
    local new_list = node.hpack(n.head, n.width, 'exactly')
    new_list.head = nil
    n.glue_order = new_list.glue_order 
    n.glue_set = new_list.glue_set
    n.glue_sign = new_list.glue_sign 
    node.free(new_list)
    end
    return true
    end
    
    luatexbase.add_to_callback('post_linebreak_filter', drop_sidebearing, 'Drop sidebearings after linebreaking')
    
    
end{luacode*}

setlength{parindent}{0pt}
renewcommand{familydefault}{sfdefault}

begin{document}    
            
dropsidebearings=1 

dropsidebearing =1 (no sideberarings)

Huge F  

huge F  

LARGE F  

Large F  

large F  

dropsidebearings=0 

dropsidebearing =0 (normal sideberarings)

Huge F  

huge F  

LARGE F  

Large F  

large F  

end{document}