We don't presently have any evidence that the encryption algorithms used by, e.g., ssh and pgp are compromised. Or rather, there have been a couple that the NSA compromised that were detected and removed. But there's no overt legal force except in a few countries like China and North Korea that attempt to restrict access to true encryption. So you could skip the writing your own encryption software part. You wouldn't want to do that anyway unless you were a top-tier cryptographer, and if you were you'd probably already have a job ... with the NSA or a private data security firm.

You can also be reasonably certain that even if your computer is internet-connected it's not compromised just by virtue of that connection, if you follow the right precautions, which are the same ones you'd use to set up an "air gapped" computer. Air gapping is more about preventing outside attacks.

Not transmitting your stuff over the open internet is not a bad idea though, if you have any concern that it might be decrypted in the future, since you can prevent it being captured and archived that way. Sneaker nets are cool.

Problem with paper mail is nothing whatsoever prevents it from being intercepted in transit, and few cyphers you could implement on paper would survive ten minutes with a cryptographer, other than a handful that are difficult and require a lot of coordination to pull off. You'd be better off mailing encrypted USB drives if you wanted to piggyback on public courier services.

But this is all silly really unless you're a superspy. Like this is over the top if you're an international terrorist. Mostly what you want is to not be casually spied on by spooks, corporations, or any random mail exchange server admin, and end-to-end encryption solves that, which anybody can do with pgp. Encryption at rest solves the problem of the actual physical hardware being confiscated, or a bad actor wherever that hardware is, or the admin who runs your mail service itself, and protonmail (at least allegedly) provides that too.