iPhone 8 battery

Hi,

I would like to ask a question. I have a 5-year old iPhone 8 and its' battery health

says "service" and it's on 75%. I am afraid that even if I replace

the battery with a new one, the phone might start crashing (actually already crashes a bit) since it's quite old

now. What do you think? Should I replace the battery or invest more money and buy a new iPhone?

Thank you.