It would be nice if it got lower, but you’re not tragically off the mark.
Others can chime in on how this theory sounds, but I’m thinking maybe you didn’t get good mixing of the extract in the kettle, and some of the extract got a bit caramelized, which would make it less fermentable.
If you were 1.030 I’d say stuck fermentation; 1.020 says unfermentable sugar to me.
Just in case though, after the beer is bottled and fully carbed, keep the bottles either in the fridge or a plastic tote. If the yeast takes off again, you wouldn’t want exploding bottles...