Resizing objects from Cartesian measurements to Ellipsoidal

I have been working on a project in QGIS 3.12.2 in EPSG 3857 so it works with google earth. Without realising the difference there would be between cartesian and ellipsoidal I buffered some points.

By default Q does this using a Cartesian system. The rest of my map is brought straight into QGIS but measurements are ellipsoidal. This means my buffer that I specified to 2km actually is about 1.5km in respect to the rest of the map. Not realising this at first I proceeded to do a few days worth of work mapping paths within the 2k buffers which now I’ve realised are all too small in relation to the rest of the maps.

I was wondering if there is a way to rescale from cartesian to ellipsoidal measures so I can get everything back in line in one mass rescaling rather than having to redo everything.