If you want the legacy workflow just deactivate interchange, Interchange.FeatureFlags.Import.FBX False.
The only difference between Legacy and Interchange is one can give you a Master Material. However it is still not fit to purpose since you seem to be either replacing the Material assignment with your custom material or copying a full graph from your custom material into the generated one…
Our goal is not to force everyone to use “our” Material Instances or even Material Instances in general. It is for the average user to get optimal material from scratch. If you are an advanced user with custom materials, at the end of the day, Material or Material instance, you are replacing generated materials with your custom ones.
And as Malakser said you can have your custom master material that is filled automatically with what is read in the FBX.