I think adding an option to choose a default value for the custom Fields would be a good improvement for the Admin. I was recently implementing a new Custom Field Set and I wanted to add default values that would be set anywhere where you would like to use the custom Fields.

I just saw that there was also another issue that was opened a few years ago. It seems this is a wanted feature and would be greatly appreciated if you would plan it.

https://issues.shopware.com/issues/NEXT-7414
https://github.com/shopware/shopware/issues/590