The SafeGuard is turning into an awesome seat for us.

If your DS can loosen the harness on the Boulevard, he will probably be able to do that on a Husky or Regent, same harness adjuster. Really, escaping from a car seat is a behavioral issue, so you really need to address that because no car seat is escape proof. You do need to make sure the harness is snug on him with no slack but not pushing into his body and the chest clip should be level with his armpits. The other thing you can do is pin the cover over the harness adjuster with a diaper pin so your DS can't lift the flap. Another option you might want to consider the Decathlon which has the same push button harness adjuster as the Roundabout which might be more difficult for your DS to do.
I totally agree with you about extended harnessing, I honestly think the very best practice is not moving a child to a booster until they are at least five, six is better. Younger, lighter children in boosters have a much higher risk of head injuries in a crash according to research done by Volvo in Sweden. Here's a link to the research paper:
Safety for the Growing Child - Experiences from Swedish Accident Data.
I can totally understand why you would want a Husky/Regent given your DS size and genetics, it is such a bummer that Huskies and Volvos aren't a good combination.

. Have you looked at the new
Radian seat? I have a friend whose 53" 8 year old was still under the top slots, unfortunately the harness was too short to do up though. The Radian is just now hitting the stores, BuyBuyBaby has them and they should be at Target, BRU, Baby Depot and TRU soon.
I know it is very $$$, but in your case the SafeGuard would probably be the perfect seat.