That is a fair point. Source first software isn’t a part of FLOSS, but even in worst case scenarios it still can be considered as “source available”. Which is better in term of transparency compared to closed source App. OP asked for “private” way to watch Youtube not open source way, so I think grayjay could still be a great choice. We are talking about watching from YouTube which is a corpo closed source spyware anyway.

Grayjay seems to be really stable compared to newpipe. I used it for 6 months now never break once. They also recently released a beta version for desktop which is quite promising

I personally think Freetube is the best for desktop right now. Recently, there is also grayjay for desktop. The UI is still buggy IMO, but also seem promising.

I am using it as my nextcloud background. Love it!