iCloud storage tiers - more than 2TB?

We have had AppleOne+ for a while now. I worked with an apple rep on why I couldn’t get 4TB of storage (it’s because my iCloud account dates back to iTools days and my iTunes account is separate - legacy issues ugh!). So my apple account has 2TB. My wife and I just turned on shared photos library and now I am out of iCloud storage space. When I click Manage Storage, it doesn’t let me buy any additional storage beyond 2TB. Is there a way to buy more storage? If not why, especially since they now enabled shared libraries?