As far as TB versus harness breeds.
Before the popularity of European WB, the TB was the sport horse of choice in North America and Britain. Obviously TB vary in their individual talents, but in general TB are fast, they have heart, they have great canters and three good pure gaits, and they can jump. The downside of TB is that they can be a bit fragile and they can be a bit hot. But also right now they suffer a marketing problem. Everyone assumes first that you can get an OTTB practically for free when it retires so they have less perceived value. And also because retired race horses can have injuries, people tend to think OTTB are more fragile than they are, when in fact they’ve held up more than most breeds would to the work. Finally dressage standards have evolved to favor the big (and sometimes impure) gaits of the modern WB.
Crossing TB on heavier breeds or ponies was very common in Britain to make hunters, and crossing TB on harness or cavalry horses created the modern European WB. So TB is a legitimate and long established part of the DNA of sport horses.
Obviously TB vary in quality for sport horse use, which is why the more quality driven WB registries want to inspect TB mares for approval.
By and large, the sport horse WB registries are no longer adding outside harness blood to their breeding programs. The KWPN is an outlier because they have multiple types including harness and heavy horse. From what I understand the registry wants to keep these types distinct. Just because you technically can breed a KWPN harness horse to a dressage stallion and register KWPN doesn’t make it a useful breeding direction.
If it was, then the European WB registries would be incorporating harness and saddle seat everywhere. And they aren’t. These are really opportunistic registries. If they thought there was a competition advantage to importing some ASB or SB or hackney or harness horse, they’d put that in the mix. They aren’t. They do use TB.
The trade off is, in general the bigger and flashier the trot, the less collection, for a number of conformation and training reasons. We already see top contender WB with 11/10 
extended trots doing surprisingly mehh canter pirouettes and piaffe (compared to say an Iberian). Most WB do not need more harness trot blood to be better dressage horses. They are already at the upper edge where they are losing the ability to collect in favor of the trot.
There’s a reason why we aren’t seeing more big stepping breeds like ASB or SB or hackney or DHH cleaning up in dressage, despite having factory installed trots that equal or exceed the top dressage prospects. It’s because they don’t have the sit and the collection. On the opposite side the Iberians have all that collection factory installed but don’t have the huge trot that is so significant in scores today.
Obviously there are big differences in gait within a breed, and big differences in conformation and ability to collect. Watching an arena full of Andalusians in hand or Friesians in harness, it’s amazing how different gaits could be, when basically unimpeded by riding, some have high knees and others have longer reach. Likewise some WB can collect fine and others are like driving a B train truck or riding a potato.
But if you are planning a breeding program from scratch you want to think about the average range of a given breed. Not just “I had a horse of breed xxx that was fantastic but an outlier” so I’m going to try to make more of him. Especially if that horse was a crossbred.
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