Screen protector is probably unnecessary, but also basically not noticeable if you have one on.
If you insist on a bumper case, most have the protector built in. I'd suggest getting one with a seal so the screen doesn't get wet if your watch gets wet since popping the case off, drying both and reattaching is annoying.
So I guess to answer your question: I suggest you go for two separate, assuming bumper case you select does not cover the screen.
The edges of the Ultra (2) ding fairly easily, but adding a full on case will increase the bulk of the watch, so hopefully you don't mind that.
Did people actually miss that? I found swipe to change faces was awful because it meant it could happen inadvertently. It's much better to just press and hold to then swap between them.
The option to swap swipe up and button functions back would have been a much better qol fix.