Thank you for your feedback and suggestions!
Some of these features are already logged with our development team for their review, and I have added the feature requests that we didn’t have filed already!
In regards to your question about the maximum number of placemarks, there really is no hard limit, as it depends on your device and it’s abilities. In the case of iOS devices, there is the option to enable (or disable) placemark aggregation, which can help with the display of placemarks as it will group them together if they are close and you are zoomed out. Android also has an option that will allow you to show an unlimited number of features, which if enabled can slow down draw time. The placemarks should not be redrawn when a new one is added – can you please confirm what type of device you are using? If possible, can you contact us at help.avenzamaps.com if you are still experiencing this issue?