Set bitcoin fee lower than minimum estimatesmartfee value
Regarding your first question, a new fee_rate argument has been added to the sendtoaddress command in the upcoming release (v0.21.0, currently at the third release candidate) of Bitcoin-Core.
In addition, a new send RPC was integrated which allows you to set a specific feerate too.
Regarding your second question (meaning of CONSERVATIVE and ECONOMICAL), see How does Bitcoin Core estimate the transaction fee? .