So Portugal was successful in sending something like 800,000 civilians to the colonies in the last 20 years of colonialism, but that was in a context of a massive emigration from Portugal (during that same timespan nearly 2 millions left for other countries, mostly France, America, Brazil), a lot of the colonists were also poorly educated and lived of purely on small trade in cities, they didn’t really contribute a lot to the economy. Also only about 1% lived in farms in rural areas.
A main problem with emigration to the colonies is that they had initially little infrastructure (but they were built very quickly in the last decades), that they were filled with disease and that they had very poor reputation from centuries of prisoners being sent there (more than a hundred thousand people were sent between the 16th and 19th century yet less than a dozen thousand lived there at the turn of the century), that means that you need modern medicine as well as an authoritarian government to bring massive investment and push people to the colonies.
The colonial policies in the early 20th century was fairly consistent and didn’t change too much despite the regime changes, your best bet is to have Portugal be scared of South African/English/german takeover of colonies and starting to settle them in the 1910s/1920s, Irl about 800k Portuguese left between 1900 and 1918, and an additional 600k during the 20 next years, the vast majority in both cases to Brazil, make Brazil less attractive and make Portugal push hard to make the colonies attractive and you may maybe send half there? The majority would still go to other American country.
After WW2 nearly 2 millions people emigrated , about half of that to France, if you already have infrastrucutre and are Willing to subsidise the trip, maybe you could send more than half of these to the colonies, in addition to the 800k of IRL, all things considered you could get 3 millions or so portuguese people by 1975 in the colonies, you gotta spend more in education to actually make them useful and make them keep their job and stay once the colonial ressource boom stops. I guess you could try to bring Eastern European in te early 20th venture and in the late 20th century Latin American and Eastern European to boost that to maybe what, 5 millions or so by 2000?
That’s not enough to keep the colonies but some small states in southern Mozambique or coastal Angola could be kept or made independant depending on the status of the Cold War and decolonisation.
As for the other way around a main problem was that Portugal didn’t care much about educating a colonial native elite, while one of the side effect was to slightly delay calls for indépendance, that also reduces link between the native population and the mainland and makes large scale immigration of African To Portugal before the late 20th century hard, after it depends on the immigration policies of the governments.
It would be interesting to see significant inter colonies migration (post loss of Brazil) , the only I know of are a few dozen thousands goans to Mozambique and Cape Verdean to angola, could be fun to have hundred of thousands of Goan in Mozambique, or a large African populaiton in Macau or goa.
If you really want hue population transfer you gotta make Brazil keep the Portuguese empire (maybe with Portugal itself too!), the African colonies had only a few million people before the demographic transition, between rebellion, famines and natural disaster in Brazil it’s easy to flood Angola with Brazilians, possibly making it majority mixed race by today. That is of course pre 1900