Best way to extend battery life on older Android phones?

I've got a Galaxy S9 that's still chugging along, but the battery life has seriously tanked over the past year. I usually keep it charged between 20-80%, avoid fast charging, and disabled some background apps, but it still barely lasts half a day now. I’m not ready to upgrade yet because it still works fine otherwise, but I’d love any tips or tricks you all have for squeezing more life out of an aging battery. Have you had any luck with apps, settings tweaks, or maybe even swapping the battery yourself? Also curious if anyone has experience with external battery packs that clip on or cases with built-in batteries and how they impact daily use. What’s worked best for you?