I just want to be able to append metadata to a Parquet file at the end without rewriting the whole file. Tombstones could be baked in the parquet file this way.

It does work with "one more file" but it's not good for performance.

This is a really clever design.

The cost estimates are particularly notable: if they're right that's a cost of about $3/day for 6TB/day of written data, 2TB/day of deletes and 50K read queries.

Storing all those TBs of data in S3 is where the real cost lies. I think it costs $5520 to store 8TB*30 = 240TB in S3, and if you retain all data your monthly cost goes up by $5520 every month.