"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 a text data in etopo?.

"project it to the same projection as these datasets" Okay, I have reprojected them and got these image data, the .TIFF files project like that also, when I input them in GIS over the ETOPO layer but I don't know how to then "add" it or "subtract 65 metres" is like a code function? cuz I haven't gotten into coding(its call GDAL, right?) in QGIS yet.