I've put a 20m buffer on all of my points (144,000 observations), dissolving all overlapping buffers.
How do I then take the centroid of the dissolved buffers in QGIS?
I want to reduce all overlapping areas to one point.
Dissolving your buffer creates a multi-part polygon. Using the tool 'Polygon Centroids' will only create the centroid for that multi-polygon. What you should do first is convert the multi-part using 'Vector > Geometry Tools > Multipart to Singleparts'. Then run the centroid tool to create centroids for each individual buffer polygon.