at the end, i would even say you should fight the final boss of the base game before the DLC, so you don't get too OP for it after the DLC and then you can go to the DLCs you only enter NG+ if you sit on the main bofire and ask to
I would beat Lothric castle first difficulty wise. Story wise, just before Soul of Cinder as the story officially ends after you beat SoC.
First DLC around levels 60-80 should be good Do Nameless King at the end game then move to the last DLC imo
and then you can go to the DLCs
you only enter NG+ if you sit on the main bofire and ask to