Do not remove the _source as this will lead to many issues, for example without the _source Kibana does not work correctly for your index and you cannot even see the data on it, so you need to keep the _source.
Unfortunatelly there is no prune processor where you can specify just some fields to be stored, on this similar post someone implemented the prune filter using painless, that could be used in an ingest pipeline, but as mentioned this can be resource intensive.
The easiest solution is to namely specify all the fields you want to remove.