Any RD'S out there? Looking for advice regarding food and metabolic issue

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I have posted about this in the past, but have yet to get an actual answer/ feed back.

TL;DR: my metabolism is about 40% lower than is typical for my stats. How do I properly figure out how much to eat without further “damaging” my metabolism?

I am ashamed to say that I have not handled this past year well. So these stats are from about 15 months ago. My BMR as determined by online calculators SHOULD BE around 1710. However, I did a full fitness panel that included 3D body scan and VOX2 assessment. Turns out, I am actually only burning about / RMR is 1040 calories daily (so about 40% less than my height, weight, BF, age, etc. would indicate. Obviously to cut calories I would need to be sub 1000 or do ungodly amounts of cardio everyday. I have tried to increase/maintain my muscle mass with weight training and increasing my existing cardio. I am so frustrated because I cannot figure out what and how much I should be eating. Even with this past years weight gain, I assume my numbers still need to be adjusted 40ish% less than my projected BMR. But if my suggested macros are properly calculated based on my weight, how do I do that?