Hi.
There is no requirement that the resources for a specific application (VM, Web App and Database) all have to be in the same resource group. You can mix these as you need.
Also, be certain that an Elastic Pool will actually meet your needs. The databases that you want to add to the Pool should all have an intermittent usage pattern with occasional burst of activity - see the description here
If are simply looking to reduce costs, then you need to:
- Ensure that the performance service tier that you have chosen is correct for your needs - reduce if possible
- Consider whether Serverless is a suitable service tier for your app - can you add code to wait while it starts up after a period of inactivity.