Sounds easy if all you have to do is write a short comment about it. People who actually did it, like the recently retired lead of the Jellyfin project, didn't make it sound like it's "not that difficult" [1].
If you want ot vibe code a project for yourself it's probably reasonable amount of effort. But if you want to build a product, something with polish, something not held together by spit and scotch tape, something reliable, it won't be easy.
From personal experience, the transcoding was the easy part. Building a UX that was intuitive and a backend that catalogued the media accurately required far more attention.
I can’t recall if Chromium supports that, I have a feeling Google only include that DRM in Chrome. But if it is available in Chromium then you might be able to build an Electron app.
Understanding ffmpeg borders on a specialization in itself. No big surprise that so many of the alternatives have fallen away over the years as people realize the absolute scope of that piece of software.
But if you don’t want to use ffmpeg directly then use one of the many ffmpeg wrappers. Or a different lib entirely like gstreamer or VLC.
Around 15 years ago I built a media centre for my car and the media playback part turned out to be the easiest part of the project.
People that run these services don't want "dynamic stream quality selection".