The floating tricks we use indoors rarely work outdoors. The closest we get is floating decks on top of fences.

The only way to get those items placed as you want is to use one of the various mods that's prohibited under the ToS.

Your choice on whether you want to take that step as other players have. If you decide to do it, talk to the players who are already floating items in their yard. No one is going to openly give you details in the official forums. Your best bet would probably be going to one of the housing related Discords.