Except within the USA, declining population growth could be reversed immediately simply by returning to our pre-1924 immigration laws (i.e. the Ellis Island era) that between 1890 - 1924 allowed the USA to have some of the largest economic growth it ever has done. Yet “conservatives” (better termed “regressives” or “reactionaries”) these days want to do the exact opposite, out of xenophobia.
That’s what the FUTO Circles project was supposed to be about, but sadly the project got ditched by the original devs for it only a year after it started. Info on that here - https://youtu.be/OKTOhxeKrK0?si=NKvI6Esp3c33DG96