Export MacOS iMessage data + run iMessage Diagnostics

This fetches data from the lastfm api, and formats it as a csv document

Enter Last.fm username, select export type and format, and click Go.
When finished you will be prompted to save a file.

This tool allows you to make an archive of your statuses, your favourites and the media in both your statuses and your favourites. From this archive, you can generate a simple text file, or a HTML file with or without media. Take a look at an example if you're curious.

Note that Mastodon v2.3.0 added an account archive download feature: "Every 7 days you are able to request a full archive of your toots. The toots are exported in ActivityPub JSON format alongside the media files attached to them, your avatar and header images as well as the private key of your account used for signing content." If all you want to do is have a backup of your data, perhaps that is enough and you don't need this tool.