It's all about tradeoffs, and what you want to emphasize in the scene.
Your instincts are good about shooting wide open for portraits, and stopped down for landscapes and using the lowest iso, but there are also reasons why you might not want to do these things--you have to weigh up what's most important to you and what you're willing to compromise on.
With ISO, you're typically going to be juggling noise against getting more light (usually to increase the shutter speed). Landscape photography typically uses the lowest iso possible to decrease the possibility of noise, but this will result in a slower shutter speed, which is why so many landscape photographers use tripods. But iso alone is not your only factor when it comes to eliminating noise--avoiding underexposure can do as much or more to eliminate noise as halving the ISO setting.
With aperture, you're juggling a
lot of different factors (which is why aperture priority is a popular shooting mode to use). The aperture controls how much light comes in, but it also controls the depth of field. And it affects
the performance of your lens. Lenses are rarely at their best wide open: sharpness, chromatic aberration and vignetting can all often be improved by stopping down at least one full stop from wide open. It's up to you whether or not it's worth it. And at the other end of the spectrum, the diffraction effect means that at very small apertures, you lose sharpness. I'm not saying you want to be at f/8 all the time. There are good reasons to just damn the consequences and shoot wide open or stopped down to f/22. Just be sure you're making a choice. You don't want to unthinkingly limit your options by automatically using only specific aperture settings.
Shutter speed typically is the simplest option--you use it to make the other two fit the exposure you want. ... unless motion blur is a priority. Introducing or eliminating motion blur depends on whether you've got stabilization or a tripod or monopod to use as well. If you're handholding the camera without stabilization, you have to watch your shutter speed so it will be at least 1/
focal_length or faster to eliminate camera shake blur and with a lower bound around 1/30s. And this is separate from freezing subject motion, which may be a completely different requirement, based on how fast the subject's moving.
I know it seems like a lot at first, but eventually, it's like driving. You will eventually get to the point where you can weigh up the situation, prioritize factors, and make up your mind about what you're going to do instinctively.