yes i am the architect. Newman’s idea is probably the fastest acting solution to spreading if in fact that is your problem. putting tension on the roof immeditely will even have some reverse effects rather than just stopping it from getting worse. cables would be SOOOO easy to do and wouldnt take any more than a good crescent wrench and a bag of hardware. you dont even need a truck for it lol. In theory putting “rafters” in would only have the added advantage of giving you some type of storage space over your garage which i could see doing for that purpose.
putting in the diagonals from the rafters to the roof is less about the spreading problem and more about helping in live load dispersment such as a crapton of snow so that it doesnt cause the middles of the roof to sag inward to hold even more snow and cause a failure.
IMHO, if its only about fixing a spreading problem and you have no need for the extra storage space do cable tension members across the width evenly spaced front to back. if its not about that then go with rafters so you have a nice storage space.
NOW, about the fact that if its wider than 16’…
the CORRECT way to connect two beams is called to “Sister” them, you overlap them about 2ft at the center and bolt them together or use a lot of good wood screws.