It comes down to, what can be done or pre-generated at build or publish time versus what must be done at runtime (such as when a viewer accesses a post)? Stuff that must be done at runtime is stuff you don’t have the necessary information to do at publish time. For example you can’t pre-generate a comments section because you don’t know what the comments will be before a post is published.
For stuff like email digests and social media posts I might set up a CI/CD system (likely using Github Actions) that publishes static content, and does those other tasks at the same time. Or if I want email digests delivered on a set schedule instead of at publish time I might set a scheduled workflow in the same CI/CD system. Either way you can have automation that is associated with your website that isn’t directly integrated with your web server.
As you suggest some stuff that must be done at runtime can be done with frontend Javascript. That’s how I implement comments on my static site. I have Javascript that fetches a Mastodon thread that I set up for the purpose, and displays replies under the post.
I don’t exactly follow your first and fourth requirements so it’s hard for me to comment more specifically. Transforming information from CSVs to HTML sounds like something that could naturally be done at build time if you have the CSVs at build time. But I’m not clear if that’s the case in your situation.
You might be interested to learn some history of societies without state-issued currency. The book “Debt: The First 5000 years” by David Graeber has lots to say about pre-modern systems of account. I’m aware there are some criticisms of the book so I don’t want present it as absolute truth - but it is an interesting on read, and it cites lots of anthropological studies.
One of the points of the book - and I see there are also other anthropologists who take this view - is there is no evidence that there has ever been a barter economy. Economics curriculum typically talks about prehistoric barter as an introduction; but it looks like the barter story may have been made up by Adam Smith. Smith’s “Wealth of Nations” is highly insightful, and even predicts problems with capitalism that we currently face. But he probably didn’t have the anthropological background to write authoritatively about economies of prehistoric societies.
Graeber does claim that there have been times when barter has been a stop-gap when there is a problem with money supply. So that’s a case where something like your app might come in,
These were temporary situations. The fall of Rome probably seemed like the end of the world to some people at the time. But new societal structures and currencies filled the gaps.