Is there any medication or supplement that would be similar to remeron (mirtazapine) as far as gastric pain relief goes without screwing with my brain chemicals?

Anti-depressants. there are many different types of antidepressants, and sometimes these will help with gastrointestinal symptoms. If you find the Remeron (mirtazapine) is working but does cause you side effects, contact the prescribing doctor, to see if they recommend switching to a different medication. Good luck.