Better luck in garage sales bro

i would recommend replacing it in authorized apple reseller like Machines/ Switch, they charges rm349 (https://www.machines.com.my/pages/service-labour-charges) but at least you dont get further issues, often people change battery from third party repair shop will lead to more issues later, especially screens.

back to your question (a) if you should replace it, i guess if you could live with it then you don't need to, if the battery is too weak and runs out very quickly then just get it replaced, or if no budget constraint then just upgrade the phone all together.

if you do it from authorised apple, they will reapply it back for you when its done.