I am late here, but wanted to share my strategies as I layer a lot and enjoy it.
First, there is a clear distinction between what I call toppers and then outerwear. Toppers for me are blazers, jackets, cardigans and such - items that serve as third pieces/completers, are full parts of the outfit and are meant to be kept on indoors. Outerwear are coats, puffers etc, items that one removes indoors. Occasionally, in milder weather, topper can serve as outerwear, but usually they are separate. Before pandemic, I wore the third piece/blazer etc. to work more often than not, so my strategies are all related to that.
For actual outerwear, I quickly realized that I need two groups in terms of fit, ones that work over blazers and ones then don't. I prefer look of sleek, form fitting coats (especially at the waist), but those don't work over blazers. When I layer over a blazer, I needed a cocoon coat, wrap coat with adjustable waist tie etc. I had to accept that not everything is going to work over everything, and to purposefully decide in advance, when buying a coat, what my intentions are so that I can get the right fit and cut through the torso.
For the third piece/toppers, which I think is what you are asking about, the situation is a bit complicated. I have three generic outfit formulas: top (blouse, shirt, sweater etc.) + pants, top + skirt, dress. All of those can be with or without a topper, often blazer. I also divide them by the fact what I want to be the star of the outfit, the topper or the underlayer.
Often I dress around blazer being the star of the outfit, in which case layering tops are super simple -usually ribbed knits in various materials (from cotton through cashmere), mostly crewnecks, that are basically like upgraded t-shirts. They underlayer can also be a simple button-down shirt or solid silk blouse, but I am generally not bothered by double collars. If the top is the star, usually when I have a printed silk blouse or sweater with interesting details, I try to wear them in weather/conditions where they would be enough worn on their own, but prepare for possibility of being cold at my desk. Often the topper is some kind of shawl/pashmina, a simple loose cardigan, a vest etc. I will layer a thin Heattech type tee under the top if it is likely not warm enough for the weather but I want to wear it on its own. Dresses are almost always outfit stars, but I do need support act toppers for them pretty often, and here I go for the vibe: moto jackets to dress them down, blazers to keep the mood dressy, etc. I also tend to generally think along the lines of longer blazers/toppers with pants, shorter ones for skirts & dresses, tough lately I have been experimenting with longer blazers with dresses, but only because that is a current trend. Sometimes a blouse or more "star" like top will work well under a blazer, too, but that is more a happy coincidence. The necklines of the blouse in such cases are high/like turtleneck, or collarless.
