Hi Gytis,

Looking good just couple of things which will help, you may have seen me post it on other threads has I seem to be repeating my self quite a it now more folks are taking to steel.!!

Make the top rail bolt onto plates, much easier for setting rails parallel and on same plane.

The connection of bed to frame looks a little weak and could flex. I'd just bolt straight to frame with angle brackets. It will also be very heavy so I'd consider having some way to help assist with the lifting, IE Acme lead screw or Large pitch Threaded rod.

Would make the sides separate bolt on sides and have the green frames separate structures, would also make the triangle braces on green ends longer so they support frame further down.
Having separate sides will make assembly easier with flexibility to adjust any error out like getting square and vertical. Also will come apart for moving or if you sell it.

Secret to good DIY is lots of adjustability.!!