Yeah those are good points. I'm spitballing here, so let's think.
Many Global South nations are trapped in historic debt that is denominated in dollars. And there seems to be this carch 22 surrounding it. For the indebted nation to pay the debt in dollars it is beneficial if their own currencies are strong enough compared to the dollar so it requires less of their currency to convert to dollars to pay off debts. But before they can pay off dollar denominated debt they have to first earn the dollars. So they have to export something. And since the US dollar is the world currency, they don't have to only export to the US. Other nations use the dollar in transactions too. But, in order to export they must be competitive. And that requires their currency to be weak compared to the dollar.
So there's a catch 22 there. A weak dollar helps with competitive of exports, but it makes paying off the debts more difficult. And the debts are designed to never really be payed off. These nations are trapped in that regard. A somewhat recent Geoplotical Economy Report discusses this, and you may find it interesting. https://geopoliticaleconomy.com/2025/02/04/trump-tariffs-global-crisis-economist-michael-hudson/
Another aspect of this is the types of goods that are being imported and exported. Global South nations do not have a high productive capacity and can only export low value commodities that tend to be raw materials. They also, due to legacies of imperialism, tend to have a "mono-commoduty" economy. Colonialsm led them to focus on the export of specific products and hence they dont have diversified economies. They can't turn these raw materials into more "processed" or higher valued goods due to their low productivities. And they are also forced to buy or import "processed" goods, like pharmaceuticals for example, from the Global North.
Now it may be that there are limitations in how effectively the Global South nations can actually build up their industry, or productive capacity. This requires not just regular commodities, but commodities thay can serve as means of production (capital goods). And although I'm unaware of the exact limitations, I imagine there are limits in place to ensure the Global South can't so easily buy the sufficient capital goods to increase capacity. This mechanism, I admit, I am unsure of. So it's just a hunch for now.
But I think that there are limitations to how a Global South nations can actually increase its capacity, due to the nature of what it exports and can import and the burdens of its debts. I should explore this in more detail as you raise a good point here.
Also, there is the theory of Unequal Exchange to consider, which further tries to explain how an extra layer of exploitation occurs here as the Global South has to labor much more (amd produce much more in value) to sell its lower priced (because they forced to be competitive) items to receive, in international exchange, less total value back in imports. Although the individual commodities it imports from the Global North are of higher value (more social labor goes into them by the fact thay they are more "processed"), in total the value of all commodities the Global South recieves back from the North is less than the value the Global South creates. The high prices (in appreciated currencies) of Northern goods ensures this.
Another issue regarding wages is that due to structural readjusting policies the IMF enforced on nations when they had difficulties in paying debts, the labor conditions of many Global South nations were forced to degrade. Public utilities were privatized, labor laws were uprooted, etc. This led to a further degradation of wages. And the low wages further help keep the price of Global South goods cheap and competitive. So the low wages make it hard, as you hinted at, for there to be domestic demand.
Capital flight is another aspect. Rich people with dollars in these countries may just store their dollars abroad for themselves, so it isn't used for productive purposes.
And there is the question of inflation. If these nations started using taking the dollars and converting it to their own currencies it may risk inflation in the local currency. But... I may be talking out my rear about these last few points. I know just enough to be foolish lol
I think some further areas to inspect is the burden of the debts (which are very large from my memory, we can look for some numbers) and the contradiction I mentioned (need a weak currency to stay competitive to get dollars to pay off debts, but the weak currency makes it harder to pay debts) which traps nations. The difficulty in any single nation in leaving a world financial system run in dollars. As well as the actual difficulties in buying capital goods to build up capacity.