Domain and forest functional level

Hi

We have three Domain controllers in our two locations.

Location1:   1. Primary Domain Controller

                    2. Additonal Domain Controller

Location2:   1. Read Only Domain Controller

All the above three servers are running with Windows 2008R2 OS and we would want to migrate the servers into Server 2019.

We would plan to install new Server 2019 and promote as a additonal domain controller and then move FSMO roles to make PDC.

I would like to know which functional level i need to set on new server 2019.

If we install with 2016 functional level and the domain controller support some of the 2008R2 servers in the network.

Thanks

Krishna