Nightmare is mainly used for it's low weight so people can put more modules and stuff on the mech without having to worry about weight limit. If you get your modules to legendary and upgrade them then nightmare will be better long term. I'd say your biggest problem with your mech as of now is low HP and not upgraded modules. You should switch zark with nightmare and then upgrade your modules and leg while also trying to get more weapons, because as you can see you have a lot of free weight already, and after using nightmare you will have more. I'd say try getting a savagery. That way you can have weapons for almost all ranges. It's a shame you rushed mything the corrupt light. You did more bad than good for yourself doing that.
You can change one of the dual engines to an armor but I thought this guy would be super annoying to fight since it has enough heat to jump back and push huggers and that its low HP would be offset by the opponent always having to jump closer lol.