I'd recommend Kiwi. You can use it combined with Wikipedia's ZIM files to have access to just about all of Wikipedia offline.

I would recommend installing all of Libre Office's programs for work purposes. It's free and open source so you won't have to deal with licenses and unwanted updates.

Calibre would be a good tool to have for reading ebooks and converting files.

There's also apps like RedditOffline so if you have an intermittent connection you can browse threads offline. I imagine similar services exist for other platforms.

Manyverse might be good for that. It's P2P social networking that syncs whenever you've got internet.

The entire suite of simple mobile apps is also really good, the Notes app in particular seems useful.

Organic Maps or OsmAnd will let you download maps offline and navigate with just GPS.

Aard 2 will let you browse an entire dump of Wikipedia and Wiktionary.

Obsidian.md

I can’t live without Obsidian. There are other similar linked note taking apps but Obsidian is the best in my opinion, not least because it just uses your folder structure and plain text markdown. I can’t imagine doing research, or just organizing my life and thoughts, without it.

F-droid has apps that do not need internet access for productivity / entertainment.

No idea, every app that involve personal relationship look shitty to me. There is no genuine share but most like useless sharing with emoticons or just few words to say nothing. Maybe wasn't too bad in stone age if we talking just about relations with other humans.