I need a new phone contract and new phone, but I need help to ensure I am not using a contract that has too much data for what I need. I need to spend the same amount or less and require a standard iPhone 11.
Optus (and your bill and My Account ) should be able to show you data usage for the phone.
You can increase or decrease your data plan or pick new plans completely anytime now - the phone bit is paid off seperately. So currently a 64Gb iPhone11 with 60Gb a month (the cheapest realistic option unless you use almost no data a month) is $100 a month for 24 months. I'd get that and change your plan if you think you need to (100Gb a month costs $10 more a month)
Of course you could save a lot buying the iPhone outright and coupling with a more targeted data plan (say if you only needed 20Gb a month roughly). Keeping your current phone would obviously further reduce monthly bills to much less again.