sql server – Using Cloud and On Premise DB together in an Application

I am developing a textile shop application in .NET Core using Entity framework. So I am developing it in Azure and using the same application for different shops using Shop ID as primary key.

In theory thats all working fine for different kinds of shops especially since the concepts is same among any size of shops.

But the problem is, in a huge shop the transactions will be very high (Traffic). So they cant accept a delay in service provisioning. So when multiple huge shops using the same DB, that could endup drastic delay in delivery for smaller shops as well.

As a solution I thought about using an On Premise DB for big hospitals and sync it with the Azure SQL. But a question arise, how fast it should sync. Because there are public users are using an app for these shops to buy online. So if a delay in sync could end up in wrong stock info for online customers.

So which is the best and effective way to handle the scenario. I am happy without an OnPrem solution, but a great configuration in Azure DB itself.

Please suggest.. Thanks in advance..