Who said anything about online services?
You do trust your Operating System, right? Then just use the built-in tools. As easy as that.
Ok, encrypt
bacon bitter goddess sheriff differ kit sock stomach rhythm skill trade drastic
with password "bla" on Windows 10 without downloading external tools, and then also decrypt it. Let me see the how-to so even my mom can understand.
Unfortunately Windows 10 don't even have feature to encrypt file with password (according to
this answer), although Mac OS and most linux distro does. But you can reduce the risks of using external tool by using external tool which are open source and can run without internet connection such as 7-zip.
And you also don't need 100-300 gibberish character, it seems you still didn't get it. Just read my last 2 posts again. You can use the same secret which in your case are a few dates. No additional characters.
You still don't get it. AES encrypting the seed words will produce 100-300 gibberish characters that you need to write down on a piece of paper, case-sensitive. Saving them in crypto.txt on my mother's computer is a bigger security risk, even though the encryption is better.
Why don't you just use BIP 39 passphrase, then save the seed words on crypto.txt without the passphrase itself?