There's still old tiny Metapad. And also more modern and fully featured (but still light) Notepad 2/3/4 and Notepad++. For full replacement, i just renamed all instances to notepad.exe.bak, back then on Windows 7 & 10, and rename-replaced it with metapad.exe. Though, i guess with UWP apps (modern Notepad is one), it's just file associations nowadays. There's surely some mass-reassociate utility around?

Btw, nano is only 50/50 chance that's it's pre-installed. Learn some vim, will ya? ;)