Holding this much of the French Empire together this long would probably require Soviet-scale internal migration and population transfers to reduce the strength of local nationalism, and they would actually have to make French citizenship feasible for at least a tiny elite throughout the colonies autonomous overseas regions. Why would local elites who would otherwise be in charge of their own independent countries buy in to this project?
At some point the demographic growth of the peripheral areas will overwhelm the center's influence, and the metropole won't be able to keep the various provinces from going their separate ways. Based on a quick, back of the envelope calculation, the French metropole (not counting Algeria) was only 40% of the overall population Paris ruled over in 1930. That rise to 51% if you don't include Indochina. If you look at France + its former colonies today, France and its OTL 2020 territories is only 13% of the total population in question, 17% if you're not counting Indochina.
This could work for some time if the French Union avoids a one man, one vote system at the level of the all-union government, and each region sends the same number of
representatives regardless of population. France would likely add a ridiculous tilt in the Europeans' favor, with the metropole getting at least 6-7 regions' worth of votes, but French West Africa being treated as 1-2 regions. Paris might be able to bribe or co-opt a tiny elite to buy into this system, but in the long run I don't see how you would avoid popular support for independence in Algeria, Vietnam, etc.