Expand the "Thumbnails: Enable for Video" options in preferences, to be a drop-down menu, so to allow the selection of one of the available modes (In Frame, In/Out Frames, All Frames, None).
Perhaps the same could be done for audio thumbnails too (personally, I wouldn't be using it, but perhaps someone else would)

I usually like to work seeing just the In Frame thumbnails of timeline clips, so currently there's not a practical way to setup this behavior for new projects, and if I really want to do it I have to go track by track changing that.

Thanks for listening!