For me, building admin tools from day one is more about culture and setting expectations than predicting future workload.
If you have the whole team aware of the basic ground rules for what the admin tools are and what they need to be like moving forward, you can build an ongoing process. i.e. don't merge a feature unless that feature's developer also included a way for the business to configure it to their expectations.
Done correctly, the admin tools should effectively write themselves. It's definitely a lot faster to do this while you are deep in whatever feature.
If you have the whole team aware of the basic ground rules for what the admin tools are and what they need to be like moving forward, you can build an ongoing process. i.e. don't merge a feature unless that feature's developer also included a way for the business to configure it to their expectations.
Done correctly, the admin tools should effectively write themselves. It's definitely a lot faster to do this while you are deep in whatever feature.