There are different size umbrella rigs. I have some that are 6 inch ones, and different sizes up to 31 inch ones. The smaller versions are mainly designed for casting but can be trolled. The larger ones are very difficult to cast. I have some that are 31 inches across with 8 inch leaders on the perimeter and 16 inches long in the center leader. No way I would try casting that.
If you are not careful with the larger ones, it can get expensive in a hurry. That's what I like about my down image sonar. I can tell a tree from a bait ball before my big rigs get into it, and get hung up. DI is well worth the money to me.
I have noticed at times trolling say northern direction I get I bite, then turn around to troll same path but southern direction and no bites. Or east vs. west. Sometimes it doesn't matter, they hit it regardless. It is trial and error each day and hour. You have to experiment to see what colors, speed, depth, direction, and area works right then. No one can say do this, this, and this, and you will catch fish. You have the right idea, though.
Same lakes on different days will be different results. Different lakes on the same day will be different.
I use different colors on my rigs at the same time. Up to 4 different colors, to give the fish a choice, then switch all the baits to the one color they choose the most often.
Is this right? I don't know, seems logical to me. At the same time, some would say, if the baits are not the same, the fish would recognize the baits aren't similar and not be attracted or scared off.
At times the fish do scare off some, when you idle over them, but normally not. They are on that structure feeding and will chase the smaller fish who are scared by your boat. Boat has bigger profile than shad which are usually shallower than the bigger fish, so they scattered willy nilly which makes the predators attack.
Normally idling over them isn't as bad as half throttle or on plane.
I am not an expert, others have other ideas that work for them. My