Premium Subscription is made for regular users, not for commercial use on another website as a part of the product.
For this use, you have to buy an API license.
10 000 sessions per day is a limit. Refresh frequency is the same as for regular Windy users (3h forecast). Check the info box in the bottom right corner on Windy.com for more data. ECMWF model is available only for internal use for an extra payment.
All the information you need about the API license is located on this page.