Err:8 http://ftp.nl.debian.org/debian stretch Release │ │ 404 Not Found [IP: 130.89.149.21 80]

You would need to use code fence. This way you could post more links :wink:
I just merged your post into one.

The issue has nothing to do with DietPi or our Beta.

This is your issue, you added a 3rd party source list configuration yourself. Looks like they removed Debian Stretch support and you would need to update source file to Bullseye.