Well, DOA6 plays very differently compared to DOA5. It's a bit more like Dimensions, in the sense that it relies more on stun then launch, rather than stun, extend stun, extend stun again, maybe just one more stun extension, launch.
I think that focusing on the triangle system first is a bit of a mistake. The triangle system while a good explanation for the mechanics, is incomplete. Why? Because it doesn't mention anything about blocking, even though it's a very fundamental aspect of the gameplay, like any fighter.
If you want good fundamentals, focus on blocking and striking first. Learn about safe and unsafe attacks, linear or circular attacks, slow or fast attacks, and their range. When you've mastered that, you can move a bit deeper in the triangle gameplay.
The triangle system comes into play when you're already a bit ahead and you get into the mind of your opponent. Basically, if you repeat the same strikes over and over, the opponent will start to hold your strikes if they know what they're doing. And that's where the triangle system comes into play. Because if you know he's going to hold you, you can choose to either use a different strike, or simply throw them. He then thinks, oh, he throw me, so next time I'll attack instead. So then you can choose to hold his attack, and then he wants to grab. Then you want to strike, and so on. Basically, the skill comes with experience. Many players use the same popular patterns when fighting, so, that helps you know what to do at a certain moment.
All that, is without accounting for other stuff, like throws beating block, ducking moves beating out highs and some mids, sidesteps, Sabakis, offensive holds, positioning (which is more important in DOA due to environmental damage)... There's a lot to take into account during fights, and it doesn't come easy to get good.
