Stop Being so Overly Sensitive
Newer cameras can produce good results at ISO sensitivity levels so high you would have had to have been nuts to even consider just a few short years ago. But your results might be even better if you weren't quite so quick to get sensitive.
In days of yore when rolls of film stalked this great land, ISO was a chemical property of the film emulsion. Emulsion layers react to light, producing chemical changes that could be made visible in the darkroom. Some films were more sensitive to light than others, requiring less light to register the desired latent image. The problem was, this increase in sensitivity was generally achieved at the expense of employing larger grains in the emulsion, producing the classic "grainy" look of higher ISO films. There was only so much that was possible chemically to boost the ISO without those bigger grains degrading the apparent resolution of the image. The smallest thing that could be resolved as governed by the grain size, so boosting the ISO was very much a tradeoff. Sometimes you had to accept the grainy look if you wanted to get the shot at all, but the overriding maxim was very much one shooting with the slowest speed film you could afford to use if you cared about image quality.
In the present digital age though, just what even is ISO? I mean, I've never once seen someone change out the sensor in their camera to adjust the ISO. It remains the same sensor regardless of whether you are shooting at ISO 200 or 2000. Instead, what happens is the base signal produced by the sensor gets amplified to varying degrees when you vary the ISO. It's like turning up the volume control on your radio. Sorry, I mean on your Spotify or Pandora or whatever. Radio is so old school. Kind of like film.
But when you boost the signal, you also boost the noise. It's all mixed together, and as goes one, so goes the other. The noise from higher ISO settings with digital comes not from the grain size but from amplifying whatever noise that may already be present. At some point, even a low level of noise can become high if you amplify it enough. "Noise" is basically pixel data that isn't supposed to be there. Regardless of where it came from, once it's there, it becomes part of the recorded image. Amplify the recorded image data, and it's all part of the same mix, signal and noise together.
Imagine you're with a good friend in a crowded room. You're trying to hear what they are saying but it's hard because of all the noise from everyone else. If you increase the volume of all the sound, the difficulty remains, but now everything is louder. What you want is to increase the signal (what your friend is saying) without also increasing the noise (what all those other folks are saying). But it doesn't work that way.
Any time you increase the ISO, you boost both signal and noise. It only makes sense then not to increase the ISO unless you actually have reason to. Sometimes, a higher ISO may be your best option. If it's a question of risking high ISO noise or not getting the shot due to darkness or motion blur, I'd take my chances with the noise. But if I do have the option of improving the exposure without pushing the ISO, I'd like to at least have that option. As such, I'm not a fan of camera Auto-ISO. If you let your camera decide the ISO on its own, it's going to do what it thinks is right, not necessarily what you think is.
Here's my typical plan of attack. Adjust the other exposure variables first, and only then mess with the ISO. Pick an aperture that will best achieve the depth of field you are looking for. Pick a shutter speed that will render motion the way you want. If the two together won't produce an acceptable exposure, it's time to consider trade-offs. You'll either need to compromise on one of these two primary exposure controls, or you'll need to boost the ISO to make up the exposure deficit.
Cameras typically do offer some option for high ISO noise reduction, but reducing noise isn't the same thing as avoiding noise. Wherever errant noise data exists, it masks whatever image data was supposed to be recorded by that pixel. Noise takes the place of, or at best alters, the image data you are trying to capture. Once this happens, all you can hope for is some magically useful means of reconstructing the correct data based on neighboring pixels.
Raising ISO should be your last resort, not your default position. With newer sensors, it's easy to assume you don't need to worry much about ISO noise anymore. You look at your results, and they seem good. Unless you shoot the same thing at a lower ISO for comparison, you can't really tell if they could have looked even better. Why risk it? Just shoot everything at the lowest ISO the conditions and your needs will support.
While shooting everything at a higher ISO setting may let you hand hold most everything, but it will come with at least some degree of additional noise. If you're goal is simplicity, this may be OK, but if your goal is to get the best image quality you can, it makes sense to stick with more modest ISO settings unless you really need the boost.
So, if you want the best images your camera can provide, keep calm, and don't be so quick to get ISO sensitive.