FFMPEG Coder - Direct Coding Ffmpeg

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 - https://www.dropbox.com/s/8sbalvl8zgik5j2/RPReplay_Final1668541290.MP4?dl=0

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:
https://www.dropbox.com/s/6fdtvchu9joznbg/93421761-F892-4E71-9307-A65E9B46B51C.MP4?dl=0

And fit it into a 1:1 square shape video, without cropping or cutting off the video.
...
By adding border/padding on the sides:

https://www.dropbox.com/s/61hqdbqqhdzsex8/video-output-C3B8CDFB-1D23-4AFC-AE24-F19D1BFCF23E.MP4?dl=0



Padding/border can be blur, solid color, or an image pattern:
https://www.dropbox.com/s/s5q4pdhxvyfmkq2/video-output-0487D39E-2815-41BF-AA73-14AD4C79B185.MP4?dl=0

User should also be able to decrease video size, but keep proportion. And move video around in the square:
https://www.dropbox.com/s/i5rvbz1rza44ncf/video-output-1CDC040B-0BDE-41A1-9BAF-CAE3D03D7DD4.MP4?dl=0

Should also be able to add more videos to the square:
https://www.dropbox.com/s/0uygiik3d35t0i6/video-output-C100429F-A4C6-43F7-BC97-A7595B29C44F.MP4?dl=0

You must be familiar with Flutter environment.
To save costs... Most likely, you will only be doing the video encoding part of this app.