The 10-second burst after bootup (note that bootup is almost immediate, it’s just the behavior shortly following the boot is slightly different) is a by-product of our QA process, and given that you don’t really reboot beacons often in production, it doesn’t affect things much.
The 2000 ms adv interval is result of our Smart Power Mode. If there are no devices around doing Bluetooth scanning (and sending SCAN_REQ packets as a result), the beacon goes to low-power mode where it reduces the advertising frequency to save battery. I assume Wireshark only passively observes the environment and doesn’t really send SCAN_REQs, hence the 2000 ms adv interval. If you do a regular scan with a smartphone or something like that, it should get the beacon out of the low power mode.
Alternatively, you can disable Smart Power Mode (: