IMO you're overthinking this.
Your bike has a hard case? Use the hard case. Nobody cares what you look like.
I actually went through the same evolution on my Triumph Bonneville. Put a Renntec rack on it and then attached a Givi-knockoff case to the rack.
In terms of "the look", it's completely wrong. The modern-looking bulbous black case is a complete mismatch to the "classic" lines of the Bonnie.
I experimented with various other options like saddlebags and even a set of side racks that would have "matched" the look of the Bonnie much better.
My first try was 20mm ammo cans (the idea being that eventually I'd paint them to match the bike.):
Then I tried some lighter weight Pelican case knock-offs:
But ultimately, I found that in terms of practical USE, nothing beat the top box. Easy to use, convenient, lockable, and big enough that if I started off on a cold morning fully bundled up in gear, and then the day warmed up (as it usually does), I'd have plenty of room to store my gear as I took it off.
I still don't "love" the "look" of the top box, but it's just so darn convenient that I've decided I can get past the "looks."