I've tried a sling type camera bag on the bike, and it was a pain in the arse. It wasn't heavy enough to risk my balance, but it kept sliding around and trying to settle under my elbow. It was quite distracting and annoying. I don't bother with it now, and just put the whole lot in my rucksack instead.

I ride with a messenger bag all the time, if you are on and off frequently it's the best solution. For longer trips (more than an hour) I like tank bags. Get a messenger bag with a cross strap so it doesn't flap around.

If it’s light you can ride with it. If not, just use cords to strap it down behind you. I am a teacher and carry a (very heavy) messenger bag, and I just strap it down behind me. When I have to carry cargo, I wear it, but it’s not optimal. If it slips, I could loose balance in city speeds.

I've done it.

I have this UTG messenger bag thing. In general wearing it on any of my bikes isn't much of a problem, except for on my Shadow where the seating position is low and you're leaned so far back that it dangles a little too low for comfort. It still works, it's just sketchy. It's one of those things where I'll do it if I have to because the computer I want to bring is already in it, but certainly not because I want it for bike specific transportation.

Surprisingly it does not flap in the wind if you are sub 100 MPH. It will slide around on its strap, though, and it swings out like a pendulum whenever you take a left turn. My commute is quite short, at least compared to the types of people who are constantly bitching about "car brain" (news flash: It's bike brain around here), at maximum about 10 minutes if I take the direct route and often rather less. As for access, well. You could, but you probably won't, and I've never needed to. You're wearing gloves for a start, and I don't know about you but I don't generally find myself needing to retrieve tools and turn bolts while I'm riding my motorcycle. Getting my laptop out while in motion is likewise a non-starter. Any of that kind of stuff you're going to do on the side of the road, at which point you will probably take your bag and at least some of your gear off, at which point it's moot.

Unless by "access" you mean getting at your gun. On that note I advise you to learn to shoot left handed first before making any decisions.

I'm not entirely sold on the idea of a messenger bag for the dedicated on-bike things. I can see how having the weight off kilter to one side like that would get old after a prolonged period, and there's really no non-annoying way to keep it secured such that it doesn't swing around. Yes, the UTG bag has a Velcro thingy on the back designed to be a belt loop for this purpose, but if you're wearing modern textile riding gear the only way to use that would be to undo it and loop it around one of the cinch straps on your pants (from behind!) every time you put it on or took it off.

I'd much rather use a backpack for that sort of thing, which explains the myriad of backpacks I own both motorcycle specific and not. Or if I'm on one of the bikes with luggage, I just jam whatever it is I have in there, and then it can be whatever bag I want.

I live and ride with a Fjällräven Greenland size S. Most excellent. It has a kind of second strap that in marketing was touted for cycling, but I find it most useful when walking a lot so the bag stays out of the way.

Quick access is the star feature. I can pull out the most needed things (wallet, phone, keys and insulin) without opening the buckle. That took some customisation though. For car / bike keys I put a ring on the shoulder strap that the keys carabiner on and off quick.

I’d get a bungie net and keep your bag strapped on the back seat. You’d be surprised how much the stuff in the bag will fuck you up if you come off the bike.

But ultimately I put a hard bag (trunk) on all my bikes. It’s so convenient.