After enabling the Xiaomi Home Secure for the camera, it is normal that there will not be the Alarm interval option when you set the Home Surveillance.
Since Xiaomi Home Secure is enabled, any motion detected within the camera's surveillance range will trigger a record without waiting for a cool-down period, so there is no need to set an alarm interval.
Note:
Low-power devices will not increase the recording time, but only the number of recording days, such as Mi Wireless Outdoor Security Camera 1080p.