Nice job! I'll keep in mind the hand truck idea you mentioned.
So far I've been able to use my platform for everything I need to move. However, if its something I need to move often I'd be looking for something more efficient like you did.
My feeling is that I will not use the strap lifter attachment very much, but it was easy to build and I could always use the quick attach plate for something else. Sometimes you have to build something and have it around to see how useful it is.
Using the quick attach plate for multiple uses is a good idea. I"m thinking the hitch adapter would be verstatile as well sinced so many things are made to fit hitches these days. I've also been thinking about a three point hitch adapter so I could use some three point hitch tools but that's last on my "versatility" list.
I was thinking of something similar. I need to be able to grab hold of a trash can and take it down to the end of a long drive. The wheeled dolly would work, but I would like it to be able to grab hold of one or two round or square trash cans without getting out of the seat.
JJ, I am attaching a picture of my trash can lifter attachment. It is for the big trash cans. Maybe it will give you some ideas. I don't use it much, but I figure it might be handy when I get a little older.
It will also lift attachments to a higher shelf.