Pattern Preview is a lot of fun. I expect you are already on top of it, but just in case:
The blue square is the outline of my 1000 x 1000 pixel canvas.
The picture elements are all on thir own layer, and made Smart Objects so you can resize without losing IQ.
The Move tool is selected, and Auto Select turned off. This lets you hold down the Ctrl key and drag with the curso over the object you wish to move. You can overlap the canvas border as you can see.
When happy go Edit > Define Pattern, but also save as a PSD file so you can make changes and define as a new pattern.
Pattern is nice but there is still a square effect. Normally the junction between hrz and vert sizes must not be noticed. This is however hard to do… only wall paper designers are really good at it