Between returns, taxes, and other stuff, I think you can model at ~55% of gross Steam sales. Then, whatever your split w/ your publisher is, on top of that. So I think it's easy for ppl to severely overestimate how much a dev is actually earning by just multiplying units times price :)
I think in general the uptick of sales volume more than offsets the discount during a Steam sale. There's a reason Steam sales have a cooldown, they generate marketing emails to your wishlists. So sales are always good, from what I can tell.
Thanks for the congrats, and all I can say is: don't give up, I'm an old-ass man and made it happen. Don't get stuck on art or music either, IMHO. Total distraction. You can make a game fun without that stuff. At least, fun enough to be sure the investment into the next stage is wise.