LibreOffice, because it is local. If I want to collaborate, I’ll share the file in whatever way is most convenient for the other parties. Since most people I collaborate prefer editing locally, this works out quite well.

If any repository that you use, or are interested in, is hosted on a commercial, for-profit service (even if it has a free tier), back it up. It will, eventually, disappear.

None, because they typicially open up a larger attack surface than the system would have without them. It’s been like that for a while now. For references, I’d recommend this article from Ars Technica, who reference some very knowledgeable people (including Chrome’s Security Chief at the time).

There was a time when AV software was useful. We’re a decade past that, the world has changed, software has changed, defenses have changed, and AV software did not keep up.