iPhone 5S stuck at battery icon

Well, no. The myth that somehow keeping it between 30% and 80% is nonsense, and is not the best way to extend battery life. Why? Because it means that all energy used by the phone comes from the battery, and none from any other power source, so the battery is supplying all power to the phone. Your goal should be to minimize the use of the battery. The absolute best way to slow the decline of battery capacity long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates.

Sorry, that’s wrong. But that’s why Optimized charging only charges to 80% overnight, then stops charging and switches to mains power for all of the overnight energy use. It charges to 100% just before you start using it, which almost immediately drops it below 100%. So it is not at 100% for a significant amount of time. The “advice” is that batteries degrade if LEFT at 100% for long periods of time. That’s why Apple optimized charging does not leave the battery at 100%. And ions don’t get “crammed"; that’s a really weak analogy. The reason Lithium batteries should not be stored at 100% is because that is the point where the voltage is the highest, and the higher voltage will degrade the electrolyte over time.

Your 8 year old phone needs a new battery. Batteries wear out, typically around 500 full charge cycles. Your battery has been running on borrowed time.