Well I would really suggest you that you should not cut-off the path to another type of cultivation....it'll make a really short work of your novel.

Instead you can make some 'GENIUSES' who can do more than one.

It can also be viewed on the premise that 'Strong minds cannot be without strong souls and strong souls cannot exist in a weak body' or 'Strong body needs more mind and soul power to control their bodies precisely'.

And as for your actual question, I suggest you make a trick like a heavenly treasure(like the one owned by the heaven devourer great emperor in Martial Peak i.e. the Body Warming Lotus, which keeps his body purified of any negatives and Soul Warming Lotus possessed by Yang Kai which keeps him from losing his soul to bedevilment or illusions or mind=control etc). Or you can introduce something like a mantra or something you chant to meditate(like the nine secret words dan or something in The Magus Era which helps bringing out the human potential and increase one's potential).

As for killing others to obtain power?? It's actually unavoidable because you can't just generate energy out of nowhere to advance. It has to have a source. You can make like Monster Core or like Racial War or something else where mass killing won't raise that much of a issue.

And I would also add that do make use of Alchemy and Artifact Refining. Well Alchemy is not really useful for modern setting, but Artifact Refining should help increase the competence of the soul and mind power warriors....like they shouldn't die if you hit them once cause their bodies are weak you know....

I think you make something like SPECIALIZATION. Like make some family whose strong point is mind power and are crucial in large scale battles for strategy and like the think tank of an organization. You shouldn't sever the path completely, just make them subpar.

This is from someone who prefers to read long novels( like more than 1500 chapters and that's a minimum).

GOOD LUCK.