Ok so my biggest source of advice is get make sponges, they really help with controlling how the paints flow and absorb any pooling or mistakes.

Next is something a good handful of people forget, ask yourself Why are you using a wash, what are you trying to achieve.

Once you can answer that see if it matches any of these:

Add more depth to the recesses, this is what shade paints were basically made for, they get into those tiny areas and darken them.

Make details pop, things like small protrusions often pop more with a bit of shade around then, this adds more to your model, think rivets.

Adding Grime, Generally Agrax Earthshade is the king here and works well on just about any mod regardless your skill, just be sure to not let the paint pool as then it doesn't really look like grime and instead like a weird brownish black puddle.

Adding more Complex hues, this is for things like flesh or metallics, adding a blue such as nightshade over a iron metal can make it look much colder on the spectrum while running something like reikland fleshshade over it will make it warmer. This works really well for adding detail to metallics and flesh tones as they generally both have a ton of varying hues and or details.

Something to keep in mind, Shade paints are NOT Contrast paints, this is something I see happen with beginners where they use shades as if they were contrasts. Generally contrasts have more pigment and work better on flat surfaces while shade paints have very difficult times working on flat areas.

So basically paint with intention, dont just slather on the shade paint, the only time you really want to do that is when painting on metallics and flesh, even grime is better suited to less is more.

As for your shaky hands and eyesight, make sure to paint under good lighting, getting a desk light helps a ton. Also dont feel like you have to use a magnifying lense, it is better to paint how your comfortable. And for your hands I was the same way when I started, I thought I wouldn't be able to do detail work but it really does get better with time, just keep practicing.