Cache video locally first? (w2gSync)

Similar to what was discussed here…
https://community.watch2gether.com/t/plans-for-local-videos/28144

Is there a way to have the video file local, on each participants’ computers, and sync the playback of that? I once found this…

…but I want something web-based and easy to use. So far Watch2Gether has been the absolute best site and I love it (bought a subscription) but…

I want to watch some long(-ish) home videos (mostly sporting events) with my family (we’re all quarantined, of course) so I set up a server and put the video file on it, and it works with w2gSync great. However, as more people joined it strained the upload bandwidth and, eventually, caused some to buffer. I compress them to be 1.5MBits/sec, but with 20 viewers that’s 30MBits/sec upload; pretty hard to do from one’s home, esp. without a CDN.

If everyone could download/cache the video first, then we could all start playing it from a local cache, eliminating the need for me to set up a “streaming” server.

Is this possible? Seems like each person would just need some indication that the entire file has been fully downloaded/cached, then some indication that each user is “ready”. Not sure what to do about viewers who join late but I guess they can just try the regular streaming way.

Thanks again for the great site.