After seeing several people ask about a tool to download their toots, I've started putting together a tool to do just that.
Masto-DL will archive all your toots into individual files (json right now, yaml to come, more as needed/requested).
This uses the Mastodon API, so will work with any instance a standard Masto client will (Pleroma, etc). ActivityPub itself does have a client-to-server aspect, so I may be able to extend this to Fedi instances more generally.
This is at present a very rough alpha; lots of debug noise and not much UI polish yet. I've tested it to be confident it won't break anything but if something unexpected happens it might not work properly. In particular, if your instance is busy/slow and returning errors the tool will probably crash.
You can check out the tool and instructions on Gitlab at https://gitlab.com/eroosenmaallen/masto-dl; bug reports, feature requests, and merge requests are welcome, as always.