Undo slider to undo loads at once
Given unlimited undo that persists across saves, undoing is a replacement for closing the project without saving in order to go back to a "last saved" state. Undo therefore needs to be made easier, so it's possible to undo lots of steps in one go.
I propose an "undo slider". This would be a horizontal slider with the thumb on the far right. Dragging the slider all the way to the left would undo all changes right back to the beginning of the project; dragging the slider a bit left would undo some changes, and those undoes should all happen visually. Thus, dragging the slider slowly to the left would "replay" the actions done to the project backwards, like watching a film play backwards. This makes it easy to rewind the project to any point within it, alleviating the need to declare "this is a point I like" by explicitly saving, and thus allows for continuous save rather than manually prompted save.