Overnight Oats as lunch

Hi everyone. I need help thinking something over. I eat flavored Greek yogurt and coffee (w/ stevia) most mornings after my workout. I realize the yogurt is full of sugar and that my breakfast is small. As I'm rethinking everything to figure out how to get off a weight loss plateau (that's now entering a year long) I wondered about revisiting overnight oats...

But I want it as a lunch option, not necessarily to replace my Greek yogurt. šŸ˜‚ See, that yogurt is 150 calories and the oats tend to be 375-500 for one meal. I'm aiming to have a good sized overnight oats some days at lunch if not having a salad or leftovers. Dinner would be my time for veggies. I am having a hard time justifying a breakfast that is over 150 calories (plus coffee) because I want to save them for lunch and dinner.

Could my meal sizes or food choices be causing my plateau? Should I just eat the same size breakfast and lunch to balance stuff out?