You can obscure multiple observations at a time on the web uploader - just select them all and then change the geoprivacy. There is no setting to automatically obscure all of a previous user’s observations in one go or all future ones, that I know of however.
If you have a home location that you observe a lot at that you would like to “obscure,” you can do this to some extent “manually.” What I (and a reasonable number of other people do), is to choose a point that is nearby to our property and within some distance that they are comfortable with their location being known, and expand the accuracy circle from that point to include the true location. You can then pin that location and easily use it for future observation uploads. In my opinion, this is better than obscuring in some ways, as the accuracy value can be much lower (ie, more precise - mine is 499 m to allow inclusion by anyone using a 500m cutoff). The observations are all at the same coordinates, so they can be IDed easily and changed in the future if needed.
There are previous discussions of the size of obscuration area on the forum. Some folks argue for larger, some for smaller. My personal sense is that a) the current size of the grid is a reasonable compromise value (some taxa might benefit from larger, some could safely be smaller) but b) there’s no momentum to change this from staff regardless.