I feel like there are some misunderstandings around the default JSON-LD export: it definitely looks complicated, but this is owing to the fact that Tropy's metadata model is rather complicated, because of the underlying customisability. For example, in CSV you typically have one row per 'item' and you have a set of clearly defined columns (title, date, etc.). Not only does Tropy have different levels of metadata (item, photo, selection) but there is not a single 'column' that is guaranteed to exist. For these reasons it would be very difficult to 'just export to CSV' in general.
That said, it is typically very easy to do so for a specific case, that is, given some knowledge how you organize items and photos (one-to-one vs one-to-many). Therefore, the best answer to your question how to export to CSV, currently, is via an CSV export plugin specialized to your data model.
I'm happy to set up such a plugin for you. Typically the best way to get started is if you take one item (or a handful of items) and copy paste them into a text editor (just select the items in Tropy, hit Cmd/Ctrl+C and paste into an editor) and then come up with an ideal CSV version of that (i.e., you need to decide whether you want one row per item or per photo, which metadata to go into which column, and so on). If you can provide me with that information, I can set up an export plugin for you that should allow you to save/export any item to a CSV file.