You don’t need DASH or HLS to seek to an arbitrary point quickly. The mp4 container format has an index which stores playhead time -> byte. This index is either at the end of the file or beginning, and tools like qt-faststart move the index to the beginning, which makes videos start much quicker when serving over http. Browsers will use the index to issue range get requests an be able to seek just fine.

Serving a 400mb video via a CDN is highly dependent on the CDN. Some will construct a cache entry from a slop of range get requests and translate them to get the missing pieces and work brilliantly and other CDNs should be avoided.