I only want to write to the free ones to extend my reach. I don't know if they are being used at all anymore though.
You will get the spam back. With filter.nostr.wine you shouldn’t need free relays. If you are afraid of being disconnected, add a few more paid relays: https://relay.exchange/ . Thanks #[5] for the list!
Discussion
Ok I check the filter.nostr.wine documentation and you just need to add the ?broadcast=true parameter and you will be posting to the free relays as well.