but have yet to figure out how to get mentions like @weirdwriter@tweesecake.social to work.

When you type @-symbol and then at least two characters, a small pop-up thingy appears after a second or few. It should appear above the word you are writing, but at least for me it appears at the top of the screen so that more than half of it is above top edge of the screen. And if the list is short enough, it won’t even be visible without scrolling the page up. My instance is running 0.18.1 release candidate, so it might be a bug in that.

That pop-up thingy lists all the users that matches what you have so far written. For example, if you type @we, words like power and Sweden are matches, because there’s “we” in them. You can use up and down arrows to select the user you want, and then autocomplete the link with enter. It won’t help you to autocomplete the link if you copy-paste the full user name. To autocomplete copy-pasted name, you need to delete the whole instance part of it and the last character of the user name. Then type the last character you just deleted, and the pop-up appears with only one choice - unless that user name exists on more than one instance. If you type the second @ for the instance part, the pop-up disappears, so you cannot narrow the list to only one user in those cases where the user name exists in more than one instance.