Glad you're enjoying it so far!
For your first question... hmm. This next update will be a huge update to weapons (new ones, making old ones more interesting) and probably the next will cover magic. After that, I'll say that the mechanics are around 70-80% there. Definitely needs more polishing, will tweak some things more, but few major systems other than summons.
Story-wise, we're probably about 1/4 done. The first 'arc' was the first leg of the journey, before the MC decides to keep going to Astrologia. I'm horrible at estimating this stuff, but it should move along MUCH faster this coming year now that I've *mostly* paid off my tech debt of shitty code and unfinished systems.
The general plan is to spend 2-3 more years on the game to actually finish the story, and then see if I'm sick of it and move on to another game, or keep going with side stories and expansions.
As for steam - yes actually, but it's probably still a bit away. I want to update the opening some more and improve the UI so I can try to make a good impression there. I'd estimate 2-3 months?