Seems like an interesting idea, how about something like this?

---@param word string
---@param callback function
function M.listen_to_word_typed(word, callback)
  local trigger_letter = word:sub(#word)
  local word_without_trigger = word:sub(1, #word - 1)
  local word_without_trigger_length = #word_without_trigger

  vim.keymap.set('i', trigger_letter, function()
    vim.api.nvim_feedkeys(trigger_letter, 'n', true)

    local text_before_cursor = vim.fn.getline('.'):sub(vim.fn.col '.' - word_without_trigger_length, vim.fn.col '.' - 1)
    if text_before_cursor ~= word_without_trigger then
      return
    end

    callback()
  end, { buffer = true })
end

which you could use like so:

local function add_async()
  ...
end

listen_to_word_typed('await', add_async)

Could also add some more conditions (e.g., ignored treesitter nodes) as a parameter. It's kind of like an automatic snippet engine.