Hi BojanZivkovic-6662,
To install the tested updates for Microsoft 365, we generally suggest defining specific path instead of CDN (Office Content Delivery Network) as update source. But it does require downloading update files which needs more cost of control.
According to your requirements, it's best set different update channels for test devices and other devices. Such as the Current Channel for test devices, then Microsoft 365 will receives new features at least once a month. And the Semi-Annual Enterprise Channel for other devices, Microsoft 365 will get new features every six months. It needs a certain number of months when some features included in Current Channel are included in Semi-Annual Enterprise Channel, during this time, you may have extensive testing.
Besides, all the update channels will receive updates for security and non-security issues when needed. These updates usually occur on the second Tuesday of the month.
For more detailed information about the update channels, please refer to "Overview of update channels for Microsoft 365 Apps" and "Update history for Microsoft 365 Apps (listed by date)".
To change the update channel, please check out the reference "Change the Microsoft 365 Apps update channel for devices in your organization".
Hope the information could help you.