What should your target heart rate be during exercise if weight loss is the goal? I have been doing a lot of cardio 5-6 days per week (running 6 miles or 65 mins on the elliptical depending on the weather) where my heart rate is in the 160-165 range. While this has gotten me in pretty decent shape, my weight loss is stalled. Would doing less cardio or less intense cardio a couple days per week possibly benefit me? 50 y/o male if it matters.

Here's a mind bender for you:  Exercise doesn't make you lose weight.  Diet and calorie intake do.  You don't burn enough calories running on a treadmill or whatever to make up for the intake.

Exercise is for cardio-vascular health and strength.

(this is what my doctor told me, and then it was echoed by the company nutrionalist.  I've lost almost 20 pounds this year, and haven't been to the gym or on a bicycle once.  All diet.)

Right on. Yup. I know. I lost 70 lbs last year doing keto and have kept it off. I weigh 190 and want to get to 175. I still have a little fat below my belly button that I want to lose.  I do intermittent fasting and eat once per day (dinner).Currently I am on a lazy keto diet with my carbs in the 30-50 g range per day. I consume between 1500-1800 calories per day depending on what I eat for dinner and if I have something other than soda water to drink with it.

Eta: a 6 mile run or 65 mins on the elliptical should burn 800-1000 calories, roughly, right?

do you know what your max heart rate is?

I ask because an hour at 160-165 bpm is probably harder than you need to go in order to burn fat

you burn the most calories and fat at 60-70% of your max heart rate

https://www.healthline.com/health/fitness-exercise/fat-burning-heart-rate

According to the American heart association my maximum heart rate is 170 bpm and my target should be 50-85% of that or 85-145 bpm. The chart has higher numbers for younger people so am I to assume my cardio fitness is that of a younger person? Or do I need to lower the intensity of my workouts which seems counterintuitive?

FPNI

This, this, this!

If you want to know your optimal HR for sure you'd need to go get tested. Anaerobic threshold, lactate threshold, and VO2 max.

But yes... your intensity is too high for efficient fat burning, as counterintuitive as this may seem

If I were you, I'd spend much less time but do high intensity intervals. Go all-out for about 30 seconds and then 90ish seconds at very low intensity... and do about 24 mins of that.

I work out with a heart rate monitor, so my low intensity intervals aren't based upon a fixed time but upon falling heart rate. (I let the HR fall to about 120 and then I go hard again)

1500-1800 calories daily for a 190lb man seems very low.  How long have you been at this intake level?  Cutting calories too much leads to loss of lean body mass and a further decrease in metobolic rate.  I would suggest decreasing the amount of cardio and adding some resistance/strength training.  
For context I am 40, 190lb and currently consuming 2500-2800 calories daily and losing weight at a rate of approximately 0.5-1lb weekly.

Congratulations!  That is some good work you've done, and I bet you feel 1000% better!

Did a 5.25 mile walk today and checked my pulse 2x both times I was right at 118-120 bpm. I'm going to mix in these workouts with my cardio and see how it goes.

Some how I managed to drop 99.5 pounds by the food I ate with doing no exercise.
Exercise just reduces your time before needing joint replacements.

Not true.

If you believe in the value of monitoring HR and using it to guide training, it's because you believe that the higher your heart rate, the more work per unit time you are doing i.e. the more calories you are burning.  So, you don't "burn the most calories" at 60-70% of max HR.

Generally, the lower the HR, the higher the percentage of energy is coming from aerobic metabolism of free fatty acids.  Generally, the higher the HR, the more calories one is burning.

When I see "FAT BURNING ZONE", it indicates an intensity where a high percentage of calories are coming from fat,  but that doesn't mean that you are burning the most fat calories possible.  The analogy is "you can have 80% of what's in my wallet or 10% of what's in my savings account".

In any case, since the body replenishes muscle glycogen - the energy substrate when intensity exceeds the ability of aerobic metabolism to sustain the work load - it's not clear that going artificially easy accelerates weight loss.  In fact, many here have posted evidence that doing HIIT, which is the opposite of FATBURNINGZONE leads to better weight loss.

Do some strength work, do some high intensity endurance, and do some low intensity endurance to optimize health, but diet trumps all.