should I replace my iPhone battery or get a battery case

I have an iPhone 11 Pro Max whose battery is dying a slow death. The battery section of Settings says it is at 89% efficiency but it seems much worse than that. Depending on my usage it barely makes it through a day.

I plan on getting a new phone in the Fall but in the meantime should I...

• Get the battery replaced or
• Get a battery case and limp along with the existing battery

I also use CarPlay and it is my understanding that most battery cases are incompatible (except for the Apple Smart Case) so that prohibits me from getting a cheaper, non CarPlay compatible case.

I can talk myself into either option above. Is it worth replacing the battery for six months of use or is it worth buying a case instead (that I might be able to sell if/when I get a new phone?)

Thoughts?