Fee rates in Bitcoin Core are measured in satoshis/kB. There are a couple places where a minimum of 1000 satoshis/kB is assumed.
Setting "incrementalrelayfee" to a smaller than default value and either leaving "minrelaytxfee" unset or also setting it smaller will be sufficient to allow your node to accept and relay transactions with smaller fee rates. Of course without the rest of the network making these changes and/or the miners being willing to mine those transactions, it won't be of much benefit.
Fee estimation doesn't distinguish fee rates less than 1000 sats/kB. This would be more substantial to change.