I would also like a way to select an entire day, but instead to delete that day's screenshots (I tend to collect a backlog of days that still need tagging and when I have the time I start at the most recent days first, so the automatic removal doesn't really work for me).

Note though that a workaround exists for the specific problem mentioned here: you can create as many AutoTag timelines as you want, so simply create a new one and name it "Default task". Then right-click the Activity timeline and pick "Add to AutoTag > Default task". In the window that opens you can then pick "Any group" and enter the default task in the "Autotag" + "Notes" textboxes.

Once that's done you can tag all your usual things, then finally use the "Copy Autotags to Tags" button in the main window's toolbar. Since there's more than one AutoTag timeline now, it will show a menu where you can pick "Default task". Note that ManicTime might show the "New tags conflict with existing tags" window, make sure to pick "Fill only untagged time" (there's also an option to never ask this again, which will flip a configuration setting for you).

Only downside is that this won't work for the moments where ManicTime is not recording (e.g. tracker service was stopped or the computer went to sleep), so you still need to pick the default task every time ManicTime shows the "Tag away time" window. If you have a habit of closing the "Tag away time" window then the "Select All" shortcut would sure come in handy.