For the last few years I’ve settled on 128GB of storage for my iPhone regardless of the model I’m using. I feel that it’s the best option for most people. With the invent of iCloud and Google photos it’s easier than ever to keep your stuff safe in the cloud.
Even when I have a lot of downloaded music I’m usually around 80GB used.
I’d argue no one needs all of their apps locally anymore. Just install the apps you use everyday then everything else can installed if/when you need them.
So check your existing iPhone and you will see roughly how much storage you need but beforehand remove those apps you don’t use every day.