Is there a way to break-down all the document to identify the biggest ones?

In case of scalars, they're all roughly the same, it's only a matter of which task reported more, so an aggregation by task_id would help you in figuring out which tasks are more costly

Is there a way to delete several :monitor:gpu and :monitor:machine time series?

Yes, these contain specific metric and variant document fields (you can look at a single document to figure out what they are), so an ES _delete_by_query request can be used to remove all documents containing these scalars. Remember however, that _delete_by_query is performance-intensive, so it will probably take much more time than simply deleting documents.

Is there a way to downsample some time series (eg. loss)?

Well, in this context, down-sampling a specific time-series is either:
Removing specific documents from that series, OR Reading all series documents in a script, down-sampling in memory, writing new documents for the new values and deleting old documents (either by query or by ID)