How hard is it to get into a good Postdoc position (or advance in Academia generally) after coming from a generally mediocre place?
The most important is not really where you do your PhD but rather if you can publish some quality research papers during your PhD. I have a few post-doctoral researchers in my team, and I have hired them because they are somewhat close to my research area but mostly because they have shown that they can write and publish good papers during their PhD. This give me confidence that they will also do good research in my team as post-doctoral researcher.  Thus, my recommendation is that if you choose to stay in your country, work hard to publish in top journals/conferences in your field, and also have international collaborations.  If you have good papers, then people will recognize your research ability.