Coming along nice Vagelis. Don't waste too much time on trying to level just yet until your ready to do the epoxy leveling. Removing twist is important and it looks like you are doing a good job here.

I would put some more supports under the Bed. Even thou your using steel it will resonate if it's not supported in the middle. I would put 2 more pieces in there.
Also like as been said braces in corners to help keep square won't hurt.

Much nicer feeling building than designing isn't it. . .Lol