I don't think that Angola is really comparable to the other examples you cite. After 1990, the MPLA continued to be the governing party and Jose Eduardo Dos Santos continued to be president (he retired last year).

It is true that the MPLA abandoned Marxism-Leninism as the party ideology in the 1990 party congress, and transitioned to Social Democracy. Also, since the 1990s Angola has accepted IMF loans and other forms of foreign investment (i.e. reconciling with prevailing capitalist/neoliberal global trends). In that sense, I would say that Angola is in some ways more comparable to China than the other examples you give, in that it maintains political continuity while changing economic policy.

On the other hand, the Peoples Republic of Benin would be an example that fits your theme of changing political leadership and constitutional reforms that abandon Marxist-Leninist ideology.

You could also point to the Peoples Republic of the Congo abandoning Marxism in 1990 and changing the official name of the country back to Republic of the Congo. Although, this example is complicated somewhat by the fact that president Denis Sassou-Nguesso did not immediately lose power, but held on until 1994 (being toppled in a civil war). Then he returned to power in 1997 after a second civil war. Similar to Angola, Sassou Nguesso's political party has historically declared itself to be ideologically Marxist-Leninist but transitioned to left-wing nationalism and Social Democracy in 1990.

Ditto, you might point to the Ethiopian Civil War, and the fall of the Derg/Peoples Democratic Republic of Ethiopia in 1991, and the transition to the modern Federal Democratic Republic of Ethiopia in 1995. However, it is again complicated that the ruling party after 1991, the EPRDF and its predecessor parties/armed wings declared themselves to be ideologically Marxist-Leninist until 1991. (that is, these were Marxist-Leninist groups waging a civil war against a Marxist-Leninist government). After 1991 the EPRDF has acted similarly to Angola in that it engages with foreign direct investment and other aspects of global finance. On the other hand, critics have drawn comparisons between China and Ethiopia as "Developmental States".

TL;DR- lots of folks and parties that espoused Marxism-Leninism in the 1980s in Africa retained (or gained) power after 1990, but altered their language and party ideology from Marxism-Leninism to Social Democratic or Revolutionary Democracy, while reconciling with global economic climate of the 1990s to present.