This is an excellent example of scope creep, <cough Bradley IFV cough>.
The original post was focused on a corvette optimised for littoral combat and this has now evolved to an AAW specialist frigate that is able to simultaneously engage 6 + targets.
Back to the original question,
@Gannt the chartist identified a couple of excellent vessels that would fit your brief and I would also add the Niels Juel class (further upmarket).
However, this conversation also ignores a key point - what is the ability of the <insert developing country here> to operate and then sustain this capability? It takes a tremendous investment in time and money to train a ship's company to safely navigate in congested littoral waters, once we add in the requirement to then fight & win in this environment the professional standards increase again (how can you fight & not collide with other ships). Now, the Navy would be sending a task force, which also requires experience in operating cohesively as a unit.This is also assuming that the supply chain is minimal and doesn't require a fleet train, which would complicate matters further. I would also note that I haven't included integrating organic air power into the equation (either maritime patrol aircraft or ship based helicopters) or, exercising as a unit against an Air or Surface threat (both of which would need to occur given your scenario).
All together to acquire and then sustain this capability will require a huge investment from <insert developing country here> over the medium to long term. IMHO identifying and then purchasing the vessels is the easiest part of the entire equation.
For example, in my alt Cambodia timeline I constructed a theoretical GDP and defence spend. Further divided this into three line items allocated for equipment purchases, sustainment (including personnel) & then base / ranges. I then used the prospective budget to work out what I could theoretically, purchase & operate then looked at training schedules to make sure that it was achievable and then sustainable.