Comment # 2 on bug 1158543 from Fabian Vogt

> Nov 21 11:54:55 tux sddm[4218]: Failed to read display number from pipe
> Nov 21 11:54:55 tux sddm[4218]: Display server failed to start. Exiting

That means Xorg failed to start, check /var/log/Xorg.log*

In any case, please provide more info - you wrote that you created a custom
.service file, but not telling about the contents.