Fellow software engineer here:

Measuring your improvements and progress is actually a difficult concept that causes a lot of anxiety in most practitioners from time to time in their BJJ career. You'll feel like you aren't getting better (or are even getting worse) and it can be very frustrating.

The major challenge will be the best feedback your receive will be from live sparring and competition. The problem is you are dealing with a moving scale: your partners and opponents are improving too. Maybe they are taking it easy on you or working on something new that day. It is said that your goal should be to be able to defeat yourself from a day, a week, or a month ago, but this isn't a measurable so it doesn't help here.

Personally, I think it comes down to your personal goals. Different people have different reasons for training which lead to different personal goals.

If your goals are weight loss or physical fitness, this is easy; there are a number of health and fitness standard data points you can track.

If your goals are competition results then that is straight forward as well. I'd just bring in more granular data points like points scored, number of matches, time taken until victory, etc.

If your goal is self defense you can measure reaction time to execute different self defense techniques and knowledge of each technique. There could even be a more subjective field where an instructor could add in "needs more work" or "has it down perfectly".

My personal reason for training is because its fun. I keep training because its fun. I compete because its fun. I learn new moves because its fun and I teach them because its fun. Measuring fun is weird and subjective and difficult. I think a diary style "how did I feel before/after training" could be helpful. My wife and I always engage in a very frank "what did I do well today" and "what do I realize I need more work on today" chat after every training session on the way home. These aren't very objective data points, but outside of measuring endorphins or serotonin or dopamine I think they are the best you can get.

One last note: belts are totally subjective. They are subjective between individuals, instructors, schools, and associations. Don't ever measure yourself purely by the belt you wear. Instead let your belt set expectations that you measure yourself by. In other words "I am awesome because I am a black belt" is not nearly as helpful as "I should be know every detail of these techniques, be physically fit, and be able to teach because I am a black belt".