What’s the process for setting a customizable background?
Are you talking about doing this programmatically or just personalizing your mobile?
If just personalizing your mobile I would recommend you post at https://www.samsung.com -> Click on Explore Tab -> Click on Community.
If this is a developer question please be more explicit
Samsung Developer Program"
I will design all aspects and functions of the watch face except the background and allow the end user pick their own background image
This is tutorial video on rather old version, but most of it about changing background on watchfaces is still valid.
Do you plan to do this in Tizen Studio? Native or Web based? If you are using Galaxy Watch Studio you can only provide backgrounds you can’t access pictures from the watch.
There may be 3rd party watch designer tools that may do this.
Samsung Developer Program
its possible find actual video tutorial? not old