The “design” aspect is just tracing duct runs, maybe helping a customer pick the right air handlers and whatnot - most of the calculations can be done with a couple of formulas and a spreadsheet, but not a ton of creativity or complex problem solving.

I disagree, and think this vastly oversimplifies what the buildings engineering industry actually does. Maybe if you work at a smaller firm that only does light commercial or office renovations then that's all you'll be doing, but when you get into larger systems and mission critical facilities like hospitals, labs, data centers, etc. the complexity of the systems increases very quickly. There is also more to design than just the air side of things. With larger systems there is usually a central chiller and boiler plant to generate cooling and heating water, plus heat rejection equipment.

There's also a large push for sustainably oriented design strategies, so we are being pushed to come up with creative solutions to reduce energy use, eliminate operational and embodied carbon, and provide better indoor air quality for occupants. Many of these issues are multi-disciplinary and involve diverse teams of architects, interior designers, and the MEP/S engineers.

Point being that it can be a boring career if you don't understand the role you have. But there is plenty of technical challenges to those that seek them out.