I wonder if you can't grab the latency correction factor from snapcast in real-time, and somehow apply it to you video stream as well? This has been raised before it seems [1], and in another issue, the snapcast author recommends looking at RTP based streaming instead of snapcast [2].
Seems like snapcast may not be ideal for this after-all :)
[1] https://github.com/badaix/snapcast/issues/57
[2] https://github.com/badaix/snapcast/issues/731#issuecomment-7...