Front disk brake squealing I know that squealing brakes are fairly typical questions but I have not found a solution yet.

My bike is a Haibike with Tektro Auriga Comp hydraulic disk brakes and in recent years I ride in southern Finland.

During past winter, my front brake started squealing a lot. This was pretty usual in wet conditions, so I didn't pay much attention and assumed the winter conditions might have an effect on it.

However, months on, I still have this problem and it is pretty annoying to wake up the whole neighborhood any time I need to brake.

Today I performed bleeding of both hydraulic brakes and also replaced the braking pads in both brakes. Rear works perfect, front feels the same as before - braking performance not so sharp as rear and squeals during hard braking and/or low speed.

One thing that's "special" in my case is that my rear brake has not worked for a year or two because I didn't want to bleed the brakes (and Finland is flat), so the front brake has definitely been used much more.

Now the first thing on everybody's mind will be brake pad contamination, but the brake pads are new and I paid extra extra care not to get them in contact with any dirt or oil.
I tried cleaning the disk with soap to make avoid any oil but no improvement.

Could it be that the disk is the problem? I read advices to make it a bit more coarse with a fine sandpaper, could that help?

I'm happy for any hints, thanks!
Presuming you were thorough with your cleaning and the rotor was left with no residue of any kind (I achieve this on rotors I suspect of being oil or greased contaminated by either using brake cleaner or by finishing with alcohol if I'm using detergent/soap), and also presuming the front caliper isn't having a fluid leak that's causing oil to immediately contaminate the system even after cleaning (pull the pads you just installed and check everywhere for oil, particularly behind the back plates where the pistons contact), it sounds like a lot of the symptoms of the front rotor being glazed.

A glazed rotor is caused by overheating and will cause both noise and diminished power. There are ways of sanding down and breathing new life into them, but just replacing them can often make more sense, especially on a basic one with some years of regular use on it anyway. You could do a simple test and swap your rear onto the front, assuming they're the same size.