I can share my current best approach to this, although I know someone has created a better one ( thus the question )
Setup
Download google drive for mac
https://www.google.com/drive/download/
Sync a folder on your computer to that google drive remote
Download and install Alfred for mac
https://www.alfredapp.com/
Download the screen capture plugin for alfred
screen capture plugin
Create a custom screen capture workflow that triggers the start of a screen cast and set it to save to the synced google drive folder
workflow customization
workflow part 1
workflow part 2
Implementation
Now, to use this workflow, I now have access to alfred to trigger the recording of the screen capture, when it saves it will auto sync with the google drive folder providing me a link to the screen cast... which I will paste into the trello card...
So, while in trello, I open a card, and to describe it further with a screencast, I trigger alfred, plug in the shortcut for my custom alfred workflow... sv - that launches a screen capture, which I use to expand on the trello card description in 5-10 seconds, and then when I save it, it auto populate