Perceived rate of exertion is a much better way to judge effort and intensity. People need a PhD thesis so we get all these fancy formulas and data, but just listen to your body when it comes to this. Also, our hearts function a little differently. I had an abnormal stress test but have a healthy, “normal” heart.

I ignore the recommended zones - they are much too low for me. According to it, my max should be abut 175-80 - in reality, my true max heart rate is 200.

Listen to your body. Do what feels challenging to you, but doable.

You need to learn what your real maximum heart rate is, ideally through a VO2 max test. Then set your workout targets accordingly. If you're using the formulas or a calculation on your fitbit, you're just flying blind and probably wasting a lot of time.

Per my Fitbit, I’m on the red nearly the whole time when I do cardio. If I try to keep it in the cardio zone, it just doesn’t feel like a worthwhile workout for me.

Where are you getting your information about recommended heart rate ranges? Most sources cite very rough approximations. Your true max heart rate is what you test it at while doing maximal effort exercise.

The number minus age formulas are not very useful in my experience.

I started going back to gym. I've remained physically active but haven't been on exercise equipment in 20 years. All through my 20s I did have a gym and I would always exercise within the recommended heart rate range for cardio. Now I am 50 and that range is so much lower. I stayed within 5-10 of the recommended max for cardio and it honestly did not feel like the same intensity that I remembered. I was sweating but not profusely or anything. I could easily talk, change the music on phone and it was pretty easy. Am I still getting a good workout?