According to Robert Gouveia (Watching the Watchers), it's a pretty broad definition, and by default nearly everything will be assumed to be an official act and the prosecution must first prove that anything that they want to go after is not, and they also can't use the president's state of mind to argue that his intent makes something not an official act.
So from the sounds of things, it's pretty durable.
How important is this for Trump?
"Core" official acts or even official acts seem interpretable or at least challengeable
According to Robert Gouveia (Watching the Watchers), it's a pretty broad definition, and by default nearly everything will be assumed to be an official act and the prosecution must first prove that anything that they want to go after is not, and they also can't use the president's state of mind to argue that his intent makes something not an official act.
So from the sounds of things, it's pretty durable.
Anything the Democrat president does is core official act, nothing trump did or will do is.