I’m not quite sure what the issue is: creating a custom smartapp with offset won’t remove the light from a Lights and Switches Shortcut. They just become two parallel means of control. But perhaps I’m not understanding something?
Meanwhile, I did want to mention that there is an offset option built in to Hello Home Actions, which might be the fastest solution.
Leave the Shortcut group as is.
Set up a parallel Hello Home Action with the offset, including whatever individual lights you like.