As far as the demo goes there likely won't be too much left to add after the April update. But that mostly goes for story content. There's still plenty of unfinished systems and mechanics that I intend to release future updates for.
And of course the overall plan is still to release the full game within this year. I still don't think the demo is in the finalized state I'd like before I go putting it on steam as well so I imagine at least one more free update over the summer at the very least.