I might be jumping to conclusions based on your screengrab, but I wonder if perhaps the issue is that most of your fields are mapped as type: text?
The available fields for a visualization will be narrowed based on the type of aggregation you select. By choosing "max", for example, you will only see fields that are date or number. Based on your mapping, it looks like the only field matching that criteria is @timestamp, so it makes sense that it would be the only result in the dropdown. I'm guessing that if you changed your aggregation to something like "top hit" you would see more fields.
I'd recommend updating your mappings, and then seeing if that gives you the results you are looking for. If not, feel free to reply and I'm happy to try troubleshooting further!
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