I'm very happy, it's working for the first time and thank you again for the good support.
I removed that parental pin code with the following guide:
Maybe by setting up the Wii U with that parental control, although I never entered a pin code.
It's strange.

Yes, I have downloaded the SLC and added that by the MLC titles.

Now another question. What should I do with the pico, it is still attached and I have to start from that?
Should I desolder the wires from the pico and connect them for example to a female header strip and tape that inside the Wii U?

Well the choice is yours. I usually remove the Raspberry Pi Pico, label the wires so that if I need it in the future I can make use of it without having to resolder all the wires to the Wii U. So I think your idea of using a header and leaving it inside the Wii U would also work.

No you don't have to start from Defuse. You should be able to unplug the Pico from power and turn the console on and have it start as it normally would. Remind me, did you install ISFShax after getting defuse to work? If you did you can either leave this on the console if you intend on modding the Wii U or you can remove it and what will be left is a standard Wii U console.

If ISFShax is installed then yes the minute menu by default it shown every time you start the console. If you want to keep it installed but have the Wii U autoboot. You can do this by using the autobooting feature in the Minute.ini file. Its part of the guide SDIO created here

ISFShax is not installed. I'm gonna take a look at that link, that's very interesting to read.
When I have my headers (thought I had some here but I was wrong), I'm going to desolder the wires of the pico and solder them to the header. In the meantime I have to think about what I'm gonna do, install ISFShax or not but I guess I'm gonna mod it.

Probably post some pictures when it's soldered to the header and let you know what I've done. 

Finished the Wii U today.
I didn't install ISFShax but continued directly to desolder the wires of the pico.
I made a sketch of the Dupont connector and named the TP wires.

Then I stuck the connector on the Wii U with some kapton tape.

Assembled the console and it's ready to play on.

In the near future I'm gonna fix another one, I have another one here that's not working and I suspect it has the same problem.

And again thanks for the help.

Nice job! I like the idea of the connector. SHould you need ISFShax you can always install it at a later date using the recovery menu and UDPIH.