s3 -> mediaconvert -> s3 -> backblaze -> fastly
gives you a super low cost solution that can scale forever. If you commit to bandwidth for fastly you can get that super cheap.
I've played around with ffmpeg for a few days, and with over 100 videos I couldn't get it to make the video smaller than mediaconvert. That's probably a personal problem, but it makes me wonder how good the ffmpeg encoders actually are. And I was using the original source to compress.
Anyway, good luck to you. The video space is about marketing. It's really crowded, and in the end marketing people don't care how much or how good the solution is...which is why BrightCove and Vimeo exist. They literally suck at everything except marketing. It'll be a great day when they die.
There are plenty of challenges with video transcoding and delivery even today. ffmpeg/libav are still at the core of any transcoding but they are opaque and their integrals are difficult to understand let alone modify. Global CDNs are able to help but you do hit a wall pretty quickly where it goes from cheap to expensive to basically impossible. WebRTC is an emerging technology that is still full of bugs in the spec and implementation. Browsers don’t really handle video decoding in what you might call robust way.
There are plenty of problems to solve in this space. Just saying that you built infrastructure that is “ reliable, scalable, high-quality, secure, and cutting-edge” is not enough to explain how you overcome any challenges. It just sounds like a buzzword salad.