This looks pretty good @argalf
I think the focus is pretty close, maybe a little bit „unsharp mask“ in image editing will do the trick. As you write in your post, optics with mirror and a big obstruction (secondary mirror) are a little less sharper then lenses. But it could also be caused by turbulences in the atmosphere, they will also make the image looking a little bit blurry. This can be avoided by shooting much pictures and stacking them. The software will then just use the sharpest parts.