It'd be nice if, once a build has been completed, the player's progress would either save automatically, or they could manually save before being immediately taken to the main menu. Because otherwise, if you haven't saved since beginning the new build update, everything you've gone through is basically undone and you'll have to start over before playing again.

This already happens, actually!

Usually a line or two before the last line in the build, the game makes an autosave, often signified by a word or phrase (usually one that means taking a break or resting) being a different color. Manual saving is turned off at roughly the same point as well. Autosaves can be accessed by opening up the Load screen and switching to the "A" page along the bottom.

Oh, it would be nice if that was communicated to the player, I've rolled back to a previous save because I thought I missed my chance lol. I always forget about auto saves.