How do I handle complex plant-wide SCADA systems?

You have come a long way. Most HMI and SCADA work no longer scares you, and that took years. The plant-wide systems still trip you because they are genuinely harder, not because you are behind. Bigger systems have more moving parts and more ways to interact. That is a step up, and steps up feel like walls until you climb them. The trick with a system too big to hold in your head is to stop trying to hold it all at once. Break it into zones. Understand one section fully, then how it talks to the next. Complexity is just many simple things connected. You already know the simple things. What is new is the connections, and those you learn one link at a time. Next time a plant-wide job feels overwhelming, pick one subsystem and map it end to end before touching anything else. Master that piece. Then the next. The whole becomes reachable when you stop staring at the whole.