I have an astigmatism, and also have a bunch of red and green dot sights, along with prism and holographic (EOTech) sights . I've tried most all of the variations to find what works best...for me.
1. Green seems to be easier on bad eyes. A slightly cleaner dot, to me anyway.
2. ALWAYS turn the brightness intensity down, way down, This still won't make the dot perfectly round, but it's be so close, that you'll never be able to tell is isn't. The dot should JUST (barely)...be visible over the target.
3. I've tried the close up mounting, and the farther out on the rail mounting. I MUCH prefer the close up mounting...despite the experts recommendations all over YouTube.
Do what feels best to...you!
Another sight that works well, is the Meprolight, M21. It doesn't us batteries. It uses ambient light to light the reticle. Round, delta (solid triangle), bullseye. I have two and like them. The dot, or lines are very crisp, but again aren't super bright, but plenty bright to see clearly. Both of mine are the Delta reticle.
Also, if you can get past the cost, the holographic (EOTech) sights work well. They have a "very", clean, round dot. BUT...you need to understand that the "colored" portion, is like painting a wall, like dabbing a sponge (lightly coated with paint) on a wall. It doesn't provide a "solid" color. BUT (again !), you should not be looking at the recital, you should be looking at the target..!
Once you get past the uneven coloring, they are very nice sights. Must be a reason the military uses them !
Mike