Choosing a GCS for a specific development (new features)

Hi all,
I have a few technical questions about the existing Ground Control Stations software.
To give you some context… I work on an innovation project which pursues the goal of developing an autonomous plane. This plane will deliver things by dropping them.
I would like to evaluate what we should use in the following software:
Mission Planner
APM Planner 2
The idea would be to add a “drop feature” in one of these.
Apparently, UGCS also has a customizable solution, but it is not open-source.
My questions are the following:
Using an existing solution, can I add new features (such as the “drop feature”?
What software would best fit my needs?
Regarding usability and number of functions, what is the GCS software you prefer?

Thanks for your help,

Did you ever identify solutions for the PX4 to UgCS software? We are currently attempting to connect our open source drone to UgCS and are not establishing a connection with the UgCS software on the computer to the drone. The drone is an Inspired Flight IF750. Thank you for any assistance you might be able to provide.