The top box has grown on me. I did not like it at first as it is harder to get on and off the bike. I am getting on in years and also only 5 ft 8 tall ( sorry, old money there ha ha). However, I like that it is lockable and secure. Mine had an issue where the pin holding the C clips that lock on the rack was working loose when bouncing along gravel tracks. I dismantled the lot, drilled through the pin with a 1,5mm bit and inserted a split pin. It does not move now. Further to that I loop a strong bungie strap through the rack and stretch it forward over the box to the hand holds. The box is very stable now. Also pack that box tight. Any free space in there and the content of the box will bounce around unbelievably! If necessary I even stuff in an old towel or piece of foam to pack it tight.Sounds like a good sized tank bag is what you need for your camera gear. I would advise against putting that in a top box unless well wrapped in foam.
I have had no problems with stability with luggage on the bike and I do cruise at enthusiastic speeds . I run Motoz Tractionator GPS on the rear in the dirt orientation and a Pirelli Rally STR on the front. Very stable up to speeds approaching the tyres rating. Great on tar and hard pack dirt. I prefer my Anakee Wilds in the dirt but recent trips have been tar biased. For me the GPS tyre is sketchy in the wet. If I do not put the rain mode on the bike spins up very easily! My favourite tyres if travelling long distance predominantly on tar or hard gravel are the Michelin Anakee Adventures.
Which ever bags you choose ensure they are well tied down. Gives huge peace of mind to know the luggage is secure when steaming along a Karoo dirt road trying to get airtime off any natural ramps in the road!