For Angola, if you are only interested about the oil, just keep cabinda, there is basis to separate it (it was separated before the 50s, and has a single, unified Kikongo ethnic group, and ofc it shortly was independnt after the Portuguese pulled off and the independantist group has been very active since), and it has 60% of Angola's oil ressources, or about, with its current population, an oil revenue of $100,000 per capita
The place was also quite underpopulated with only 80,000 people in 1970, many of the portuguese settlers came over the past 20 years, so they didn't have too many link to the land, i think it would be easy to convince a good hundred thousand or more (from a population of 700-800k in 1974 in mozambique and Angola) to move there instead. If there isn't *too* much corruption, it would likely become very wealthy, potentially contributing to 1/3rd of portugal's exports (about 45 billion per year in oil, while portugal's are about 85 billion already). Currently IIRC there are over 20 years of reserves at the current production rate, with large scale international production having started about 15 years go. $1.5 trillion over a generation will change a lot of things for portugal, maybe not in a good way...
The place would likely attract a lot of people, wealthy white (and asian) people, who would help sustain a white majority of "legal resident", but also a shitton of african migrants, the second congo war will be especially interesting as there may be a 1:1000 difference of gdp per capita between the two bordering countries... the entire 300 km long borders will likely be heavily protected.
Then its place as a haven of wealth and stability in subsaharian africa could attract lots of investment, and be the center of many african companies, communication for exemple. If it plays its card right i don't even think it's impossible for it to surpass the mainland's GDP per PPP (by comparison with Kuwait), although at this point the possibility of independance can't be ignored, and i doubt Lisbon would want that.