FFMPEG Expert Needed...

IF YOU HAVE NEVER WORKED WITH CUSTOMIZING FFMPEG, DO NOT APPLY TO THIS JOB... I am creating a hybrid mobile app that can take a rectangle video & display it in different ways. Here is the current app - [login to view URL] I could not find suitable SDK or libraries currently available do the features that I want. I need someone that can custom code this. So I need someone that can customize it directly in ffmpeg api/module. The app must be able to do the following: I want to be able to take a rectangle shaped video like this: [login to view URL] And fit it into a 1:1 square shape video, without cropping or cutting off the video.
... By adding border/padding on the sides:
 [login to view URL]

 Padding/border can be blur, solid color, or an image pattern: [login to view URL] User should also be able to decrease video size, but keep proportion. And move video around in the square: [login to view URL] Should also be able to add more videos to the square: [login to view URL] You must be familiar with Flutter environment. To save costs... Most likely, you will only be doing the video encoding part of this app.