I've been doing BJJ for 3.5 years now. The teaching style is a complete 180 from what I'm used to in TKD.
- TKD: Each belt has a set number of techniques, combos, forms, and self-defense sequences that need to be learned for the test; or has a lot more forms and then is more open about the approach to technique and self-defense. Every class you can expect to drill kicks and practice forms; and you'll probably also drill hand techniques such as blocks and/or punches. You'll likely drill dozens of different techniques and practice multiple forms every class. You can also expect every class as a white belt to be the same as every other white belt class, every class as a red belt to be the same as every other red belt class, and so on.
- BJJ: 95% of the art is open to kids, 97% of the art is open to white belt adults, 98% of the art is open to blue belts, and everything opens at brown. Most classes are focused on white belts of the age group. Every once in a while, blue belts will get instruction on wristlocks, but most colored belt techniques are left for 1-on-1 training. Each class goes into 1, 2, or 3 techniques that you spend 5-10 minutes each on, and then you do various sparring games to work on the position.
I know it's possible to do BJJ more like TKD. There are some BJJ schools that do this: have a codified curriculum for white (and maybe blue). Roy Dean even has his students go through "crucibles" which are belt tests for every belt color, although upper belts it's more them running a seminar and technical demo than a traditional TKD test.
However, I'm curious if it would be possible to teach TKD in this way. Only teach 1, 2, or 3 technique sequences per class. Maybe even have rank progress similar to BJJ: there's no test, you just get a stripe or a belt when the Master determines you're ready based on mat hours, time-in-grade, progress, ability, attitude, etc. This style would likely do away with forms (because long-term memorization of specific sequences would not be the goal or method) and focus on understanding of the technique and the concepts. Each class would be a deep-dive into something like a 1-step sequence, kicking footwork, striking combination, or something to that effect. And then after drilling those, there would be a specific sparring session (a semi-scripted spar where you train the situation against resistance), and then free sparring using normal sparring rules.
This isn't the direction I'm planning to go. It's more a thought experiment.