Today it was a sunny day, I was driving all day around until almost midnight.
I just compared the SJ4000 recordings with the Xiaomi's.
First of all, I can say I have no focus problem now. I didn't drive more than 30 minutes each trip but the dashcam was left in the car all day, in the sun, so it was heated by the sun, cooled by AC and again. No focus degradation.
Second, during the day, the image is brighter then before and I think sharper. I compared many minutes of video SJ vs Xiaomi, the image is too sharp, small details cannot be understood. Now I don't remember how it was with the original lens, I will put them back in few days to check. But maybe if I will de-focus a little the lens will be better, I will try.
And third, the night recording is better then SJ, more clear picture, the road signs are visible and understandable. This was also before, with the original lens, so I guess there is no change in night recording quality, except the focus, of course.
Changing the lens is very easy once you opened the camera, just remove the glue and unscrew the optical block of lenses. Better put a mark on it before unscrewing it, just to remember where to set it if you put it back, like a scratch on the lens ring at the vertical position. The difficult part is to find another block of lens from another camera and be compatible. The SJ lens screw has a slightly different step then Xiaomi's, but it only needs two turns to focus. The original needed about 6 or 7 turns to focus.
You need to use the app, better on a large tablet, to see the image when you focus the lens.