To be honest I shelved this project months ago. Recently, I've been thinking about going back to it and finishing it up. Your comment gave me the push I needed to actually open the project back up, so thank you for that :).
The multiselect feature you mention is actually implemented but I did not push it to itch, because it's not polished. I also added an option to spread pieces in a circle or a grid, but it's work in progress as well.
Some - like sorting trays - were in the game at one point but I scrapped them. I moved the whole puzzle to the 'board' and you can just sort it there. I will probably add some kind of sorting zones you can draw on board to have a visual separation.
As for the full release, I'm really happy to hear you think it's a "good, high quality game". I believe that for Steam release there is a long way ahead with fixing bugs and polishing things up. Also the interface is very meh and I'd like to redo it at some point.
For now I'm picking the project up and will continue to work on it. New builds can be expected to be published on a somewhat regular basis on itch.