In Lightroom Classic CC I wish I could drag out a graduated filter (or a radial filter) then right click on it and choose “Convert to Adjustment Brush”. I could then manually add to or subtract from the adjustment mask.
In either one of those tools, after you have applied them, there is a brush that you can choose inside of the toolset. One of the options is to erase, and you can erase parts of what you have applied with either one of those tools. Wouldn't that accomplish what you are requesting?