https://docs.qgis.org/3.34/en/docs/training_manual/processing/no_data.html might be helpful for using QGIS to add two layers and a constant together....
I didn't have an exact sequence of steps in mind, just in general that's how you'd have to approach the problem, and indeed that may require putting...
"so you'd need to take current bedrock topography data of these areas (think that's included in the etopo1 dataset)" sorry, can you elaborate?. Like...