This is working great -- didn't know subscribed had a microblog feature, thanks.
I've been playing with subscribing to gup.pe groups as well, and they show up in subscribed microblog. E.g. subscribing histodons@gup.pe pulls in most mastodon posts for history.
Others have already mentioned other cloud storage options, which are good because they both synchronize your writing and also give you a backup in case something goes wrong.
If you already have a backup solution, you could consider syncthing. It keeps folders on several different devices up to date without storing a copy in the cloud somewhere. It's free but takes some setting up
You could also compress your files into a zip, add a password, and then store it in google drive. You can't do online writing and it'd be a hassle to re-encrypt them each time you make a change but google wouldn't be able to scan your docs and you could restore your writing if something goes wrong on your local computer