The Proto D21G has been a solid product and combined with Microsoft VS C++ (free) & the Classic VisualMicro Plugin, makes a solid platform for development. The VisualMicro plugin can be downloaded (via extensions) for free and their cost ($45) for subscription is very reasonable. If you have not tried it, I would give it a whirl. While the Arduino IDE works for simple projects, it quickly falls short on more complex ones. Note: The intelli-sense features alone are worth the money.
Microsoft has made available a GDB option with their android development tools that allows even better debugging (vs classic VM debugger) when using the SAM D21 (installs with VS). Unfortunately, VisualMicro currently only has definitions for Arduino Zero, Arduino M0 and Texas Instruments LM4F.
They do have instructions on how OEM's like yourselves can add board definitions to allow this debugger to function. This information is way over my head and I'll would need your help to implement. I've got to believe other users of your product would benefit from such a venture. Is this something you can help implement?
Main Site For VM Plugin
General Infomation on GDB Tool
Comparison Between Debugging Options under VisualMicro
Site to get VS 2017 GDB Plugin
Site to get Microsoft VS Community
Please try to give a substantial answer. If you wanted to comment on the question or answer, just use the commenting tool. Please remember that you can always revise your answers - no need to answer the same question twice. Also, please don't forget to vote - it really helps to select the best questions and answers!