When editing photos on a MacBook, is there a way to discard all recent changes only?

Try ⌘+Z to remove the last edit. A second ⌘+Z will remove the next to last edit, etc. Not sure how many levels you can go with that but I've done 3.

You can revert all edits individually by disabling them in the Adjustment panel.

Just click the bug checkmarks to remove an adjustment individually.

But if you are trying out adjustments and want to remove only the trial adjustments, I would always duplicate the image with ⌘D and then try out the edits on the copy. This way the current edits will remain unchanged and you can simply delete the copy of the photo, if you want to revert to the current state.

The duplicate does not need much extra storage, because the first version and the duplicate will share the original file.

You'll have to keep track of the number of edits you perform on the image.