Akira (Sly Bass) & The Elusive 246P

No, you can do 64H+P, 2H+KP, (2)46P on all characters when it comes to open space/flat surface (also on Gen Fu who is considered heavy weight) exept super heavy weights (Bass, Bayman Leon, Rachel). If you don't connect the (2)46P against mid weights is because you buffer it too late or your imput is too slow, I don't know.

There isn't a tip to be more consistent using 66 before 2H+KP. It's all about timing...you have to press 66 immediatly after 64H+P...then the dash must be short but not too short (and if it's too long 2H+KP won't juggle). You will get consistence trying it in training... But you are right, it's a bit risky. I usually prefere doing 2H+KP, (2)46P instead of it. Anyway when you have a wall behind you you can do 64H+P, 2H+KP, 2P, 466P+K (you can end it also with 6PP or 46P if you want a better recovery) without the dash before 2H+KP if you get the right angle. But if the angle you get doesn't allow you to connect with 2P just do (2)46P.

That throw combo only works on feather weight and lighter when it comes to open space/flat surface. Busa is a mid weight.

I'm aware of the other technique you mentioned. @DestructionBomb pointed that out to me a while ago, but I can't rely on that due to inconsistent results on 66 getting you into the proper range.

If you have any tips on how to achieve consistency, I'm all ears.

You don't need a downhill lie to do 64H+P, 2H+KP, (2)46P... you can do it also in the Dojo stage for example if you want (it was possible also on DOA5). All you have to do is buffering (2)46P during the 2H+KP animation. This is the most damaging combo with no forward dash before 2H+KP you can do with Akira after that throw. But the most damaging is: 64H+P, 66, 2H+KP, 2P, 466P+K.