High-Quality Looping Transparent GIF Animation Creation from MP4s

I need a professional to create high-resolution, transparent looping GIF animations from two short MP4 videos (7s and 10s respectively) to be used as full-screen backgrounds on a website. These animations must seamlessly loop, have their black backgrounds removed, and be exported in multiple resolutions optimized for both desktop and mobile.

Summary of deliverables:
- The black backgrounds of the videos need to be completely removed. The final GIFs must have a transparent background to overlay our site's content.
- The animations must be seamlessly looping, with no visible start/end, ensuring a seamless loop without any visible breaks or interruptions.
- We need two output resolutions for each GIF (desktop and mobile) (at least 1920x1080 for desktop and 1080x1920 for mobile).
- The GIF durations should be about 6s and 9s, matching the usable portions of the MP4s.
- Deliverables must include both the final optimized GIFs and editable source files.

Possible steps to complete the task:
- Import the MP4s into Adobe After Effects.
- Use the Keylight 1.2 effect to remove the black background and clean the matte.
- Identify and trim a loopable section of the animation (~6s and ~9s) and pre-compose it.
- Export a transparent master file (ProRes 4444 or PNG sequence).
- Convert the master to a transparent looping GIF using Adobe Media Encoder or Photoshop.
- Export two versions: one for desktop (1920×1080 or larger), and one mobile-optimized.
- Provide all final GIFs and working files as deliverables.