Still Arch on main desktop, but slowly moving towards NixOS everywhere.

That mindset unfortunately leads you to being locked into vendor-specific ecosystems with no control about the software you're using. The big vendors (MS, Apple) know this and have already started extracting more value (in form of data) from their users. Next step will be to put more stuff into their clouds and sell you a subscription. You'll be renting software with included spyware then. With zero control yourself. Linux and FOSS gives you control back. It's also quite easy to use in 2023.

https://github.com/libre-tube/LibreTube refers to https://libre-tube.github.io/
Any other site is probably a scam.
Invidious is nice but it probably leaks your IP address to Google/YouTube, which you don't want when using a privacy-friendly frontend for it. Especially not if you use your real IP. The Piped frontends, on the other hand, don't leak your IP to Google/YT.

Highly unlikely. A site could try to exploit unpatched security holes in your browser, but if your browser is up to date, this is unlikely to succeed. Modern browsers are very complex and large so they have lots of weaknesses, but they also get fixed quickly, a lot of eyes are on their code and they utilize sandboxing techniques as well to isolate things from your system.
Still, it's a good idea to harden your browser further yourself, or run it in an additional sandbox.

Check Flatpaks as well.

There will not be the equivalent compatibility at first. But there will be enough compatibility for most users to not care about it anymore. The equivalent compatibility will only exist once literally every hardware and software manufacturer supports Linux on their own as first-class citizen. And they will only do that once Linux has signifikant desktop marketshare. But it doesn't matter as long as most stuff still runs no matter what. Which is currently the case. And it's also gotten easy.

Technically there are still workarounds like disconnecting from the network or editing the installation sources, but it's still anti-user and worse than in older versions. Win will continue to get worse over time. Look at a freshly installed, default W11 Home consumer desktop for example. What most people probably use. Just open the start menu. It looks like the OS needs an exorcism first, before you can use it. But maybe many people have already become used to things being this bad

"Long way" won't be long, because Google 2.0, err, MS' direction continues to make Win worse over time (cloudify everything, extract more data and strip more rights+control from each user, and gain more money via price-increasing subscription models) while the open source desktop ecosystem around Linux is getting noticeably better for almost every user every ~5 years or so. The era of Windows as a "pure" OS died with W7. Since W10, it's OS + integrated malware. Start of downfall.