Thank you for sharing your experience with us @Meeples! Also, excellent work covering all those steps. With this concern being intermittent or inconsistent in when it occurs, checking connection is the best approach. Intermittent connection to live view is often an indicator of a fluctuation in resources. Some variables that fluctuate by nature are wifi, mobile device connection, and power (battery).
Having an optimal battery level will, of course, ensure that your device is connecting the quickest it is able to. Testing on wifi only, and cellular data only, was a great step to take and is recommended to try periodically in order to observe differences in results depending on where you are or how you are connecting.
As the network speed package and signal strength sounds sufficient, and your Video Doorbell is connecting just fine, this is the last variable to check. If you find yourself needing to check your network further, your best next step would be checking router settings and your ability to stream on multiple devices. I hope this helps! 