@Michael Weis
A few things here:
1. David Green's recipe works for out of state, not international. Trying to apply US recipes to Mexico generally won't work.
2. I'm not sure why you reached out to Sotheby's. Their properties aren't the best for investment (unless you're a high net worth individual) and they generally focus on lifestyle properties if I'm not mistaken.
3. I don't know what market you looked at (although I think I guessed) so it's difficult to comment on the competitive situation. Generally speaking, the Mexican market is way less saturated than the US market but there are exceptions and a 3 BR+ property won't save you there.
4. When you mentioned studios and 1 BR properties taking in upwards of $500k per year, I imagine you're talking pesos, not dollars.
5. I imagine that, in every market, there are listing with 85%+ occupancy. They're the best listings. With no experience out of your area and in STRs, I'm not sure that's the kind of occupancy you should be targeting.
I'm not sure there's a right agent. Working with an agent in Mexico is not the same as in the US. Most have only access to limited inventory, they don't have the same fiduciary duty and you have to make sure you don't get taken advantage of. I've never used an agent to buy properties there. I invest in developments nowadays but, even when I was purchasing properties, I did so directly from the sellers so I only dealt with agents on the sell side.
If I was in your situation, I would find the properties and then contact the listing agents. This way, you wouldn't depend on solely one agent to find properties and you wouldn't be overly dependent on advice that might be biased.
Hope this helps