I think your site structure here will be essential in whether this project is successful or not.

Without knowing all of the details/just odd the top of my head, I would recommend breaking down your guides into location silos, something like site.com/country/field-guide-item where each field guide item is the topic you're discussing, such as "best restaurants in XYZ country" or "top water activities in XYZ country," making sure that each field guide item is based on keyword research for the topic at hand.

From there, you need to make sure you have optimized:
-Titles
-Meta descriptions
-H tags (as you've already mentioned)
-Images
-Breadcrumbs with schema markup
-Internal linking within the country field guide silo

I would not combine multiple areas as this will confuse google about what each page is about, so follow your gut there. If there is only enough to do in an area to write 500 words, then so be it!

I hope that helps as its based on not knowing your site and structure plan. However this should be able to get you started in the right direction. :)