This may not work for you, but helped me for a bit in HS.
Don't use a desktop environment. Literally run in text mode. Use Lynx/Links/w3m if you need to look something up. VLC has an ncurses interface, and you can use youtube-dl to download music. Use emacs or vim for text editing, and tmux to split the screen ( + more, but I never used it to it's full potential ) and you can cover a lot of ground.
For the innevitable need to view pdf's and pictures, download them to this text mode machine, and then file transfer them to an offline only tablet.
Did I discover this because I was dumb and didn't know how to handle nvidia drivers? ...I was a noob ok :| . That being said, I think I focused more and accomplished more with this setup then I did for the next 3 months after I set up a DE.
100% offline is basically impossible, but very limited and high latency (we had a 3 second ping) is doable. Mostly you have to remember to download things when you get the chance. Get offline copies of Wikipedia and stack exchange (via Kiwix), download package docs as PDFs.
Installing anything is a nightmare so work on a cloud system and run a job server if you're able to (eg jobs that take hours to days and need a connection). Pip is a disaster for this - download every wheel just to check the toml file for dependency resolution? Just doesn't work with poor connectivity. Get good with rsync and wget flags to manage partial downloads. My preferred method is to pull remotely and then rsync partial to a local machine over a few days. That's much more reliable than wget with the -C flag.
You end up building a lot of packages from source, a shallow git clone is far smaller than a compiled binary.
Forget Overleaf. Install tex locally like the old days.
Also I remember reading an article by a guy who went to teach computer science in North Korea - so yeah, he had to be an academic without internet access. His main point was that it just makes you remember that books still exist.
I know some mathematicians print all the supporting material they need in advance and then work on paper for weeks without internet. Although admittedly that's a fringe option. And contrary to the expectation that they will probably need to look up things that come up later, they are actually very productive. (That's just an anecdote from second-hand, as in: I know a mathematician who knows a mathematician who does this.)
For my own coding, I usually don't use the internet anyway. I browser offline documentation of the stuff I use, vendor all my libraries (i.e. building doesn't require internet access), so maybe I'm going to try disconnecting my computer during those sessions. That's of course not counting coding at work, due to the tyranny of Slack.
