It seems like you’ve got it covered. The only other tip is to maybe take it a bit slower than a blind wholesale update. You can always update the packages or dependency chains individually with emerge -DNuav <atom> instead of world (use –oneshot for things you don’t want to add in to the world file, like system libs for example).
Note that it’s always easier to do if you keep up to date regularly rather than going for monolithic batch updates every couple of months. Also, to avoid too many surprises, you might find it helpful to subscribe to one of the mailing lists like gentoo-user (WARNING: ~100 mails a day on this list) since pretty much any major update issues are likely to show up on the list with plenty of discussion explaining how to solve them.
I would guess that the next major “problem” update will be the migration to openrc once it goes stable (it was done on ~arch middle of last year iirc with only a few issues, primarily with networking changes).