TubeCast is not a full Chromecast implementation nor will it ever be. It only supports the first revision of the Cast protocol and will only keep working while Google keeps backwards compatibility with the Cast V1 protocol in their youtube mobile applications (at least for service discovery).
This disclaimer doesn't sound too confident, but if it works for op, then it should be good in the meantime.
I'm using macast. no recent development but works fine for what I need it (play full-screen video with mpv when sent from mobile device). on the phone you need allcast or similar app that you share the e.g. youtube video to.
I don't have an answer but I am piggybacking off your question:
Is it possible to flash another OS on the last gen Chromecast? Considering what's happening to the gen 2, I don't see a very bright future for my Chromecast.