The best time to replace the battery is when it's causing you trouble, in the form of either performance issues or poor battery life. The 80% figure is meaningless, that's just what apple says your battery should be at after around 500 charge cycles, but that varies with usage.

I've seen 4 year old iphones @ 73-75% battery health still getting very decent SOT, and I've seen 1.5 year old iphones @ 90-92% battery health getting absolutely shit battery life.

Is the phone not lasting as long as it used to the point it's interfering with your day to day activities? Change the battery.

Is the phone more laggy or stuttery than it used to be, perhaps getting ocasional freezes that bother your? Change the battery.

Ignore the battery health indicator.

You're saying the metal piece is from behind the earpiece speaker, yet the phone op showed us in the photo looks intact and absolutely identical to the one in the photo you keep repeatedly posting in this thread.

As a self-proclaimed phone repair hobbyist can you explain how that metal piece that is clearly larger and thicker than the gap between the display and the borders of the phone somehow came from the behind the earpiece and got outside, yet the phone in OP's photo doesn't appear damaged in any way whatsoever.

Again, look at the phone in OP's photo, and the one in your photo. Despite his photo being slightly blurry, you can clearly make out that both earpieces look the same.

Wrong, the rear part of the earpiece speaker for the 13 pro does not look anything close to that. There's videos on the internet that disprove you, there's common sense that disproves you, you can watch a jerryrig teardown of the phone and it will disprove you, but it seems you're so adamant on your stance, despite it being demonstrably wrong, that I sincerely doubt you'd believe apple's own engineers if they told you it's not true.

That metal piece did not come from OP's phone.

Because he's wrong. That metal piece did not come from the phone.

That is not the earpiece mesh, the earpiece mesh can clearly be seen in the top photo on the phone.

The earpiece mesh is exactly that, a mesh. The thing OP is showing is a piece of solid metal with some holes in it.

Definitely not the phone. The phone in the picture looks perfectly normal. Moreover, there are youtube vids showcasing a teardown of the 13 pro. Wherever that piece of metal came from it's 100% not from the phone. Your 13 pro is fine, nothing to worry about.

That absolutely did not fall out of your iphone 13 pro. That is not a part of the phone.