I'll echo DA… hard luggage is best (secure, waterproof, theft resistant and large capacity) and doesn't have to be slow to mount/ reconfigure.
1. For my daily "light"use: I have a Givi E52 Maxia top box (big enough for two full face helmets) with a luggage rack frame added to the top to bungee excess items.
It's currently mounted on a Givi monorail but I'm intending to swap to an SW-Motech alu rack (not the one for our birds but one for the Suzuki Bandit.
It fits too but allows the grab rail to remain in place. It also sits further back which will allow clearance (as does the monorail) for my Beetle Bag lids to open ok in my next config).
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Bandit rack on the left, Bird on the right (though without Givi adapter plate). Pics borrowed from Mikeyw64's post in the "Neatest top box mount" thread.
Neatest top box mount?
2. Town load carrying: as per 1 but with Corbin Beetle Bags added (on quick release lockable mounting bolts). Medium capacity, oddly shaped interiors as they are scupled to the bike but slim enough to filter through traffic (wing mirror span wide) though watch out for top box clearance re their lids. Very aero dynamic too.
They aren't made any more so can be hard to source (ensure you get the mounting brackets too as they are rarer than hens teeth). The locks are on the top and prone to rust (spring failure) in wet weather locations (easily replaced though with higher quality ones plus weather sealing caps).
One downside is they are overly heavy (though very "solidly" built out of fibreglass) for their limited carrying capacity.
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3. For full touring I swap out the B bags for Givi E41 monokey panniers mounted on quick release lockable SW-Motech side racks. Mikeyw64 posted an excellent vid showing just how rappidly you can add/ remove this set up. So to avoid a thief doing it too the locking bolts are sensible additions.
4. Final for even further additional capacity (if I'm one up) I swap my dual seat for a solo Corbin Smuggler which gives easy access additional "underseat" boot/ trunk space.
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5. Final point: I've swapped out the three original Corbin barrels out of the bags and seat locks and replaced them with the Givi monokey barrels so all my luggage is on one key (not potentially six keys). This helps avoid walking around like a prison guard with one humongous bundle of keys (a point to bear in mind with lots of luggage esp if from differing manufacturers).