I think a linear hydraulic actuator could work very well, though it seems most are very large and relatively expensive for what's actually needed. This one seems like it could be good though:
I also think a magnet could work well. The only thing I'm still struggling with is how I could mechanically seal the bottom lid again. Technically, the user might never need to reseal the bottom lid until the mycelium has already fully colonized the bucket and it's time for them to open it up, so leaving the lid open until it's manually closed again may be a non-issue.
The only part I need to figure out now is how to add a hinge to a circular lid that attaches to the circular tube. The closest thing I can seem to find is this, though the way the hinge attaches to the tube might be a little too flat: