On the topic of when to break a horse, I don't like to start them until they are at least 3. Even when a 2 year old is large, it doesn't mean their bones have finished growing or are strong enough to start carrying a rider without issues down the road. I never understood why TB trainers start so young. I would guess that has a lot to do with why those TB's started at a young age have a lot of leg and foot problems?
I also associate "tank" with a really bulky horse. I'm not used to the more "streamlined" breeds.