Commuter Bag

Looks pretty good. Only issue is the hip belt has zero padding and that's where 80% of the weight should be borne, especially if you're carry all that stuff.

And I prefer having small pockets on the hip belt to store keys, security card, phone etc. That way you don't have to take of your pack every time you have to access doors, house, bike lock, answer phone etc.

I found carrying all that on my back was not good for it.

What worked for me was rear rack and panniers. Sure, a bit less aero but I arrived with less heat stress, no stinky sweat drenched backpack, and no back pain.

And rack and panniers is worth extra points for commuter cup racing, especially when you pass roadies going Parriwi Rd

How quick are the panniers to put on / remove?

Will likely start on my do it all MTB and then perhaps move to a road bike later. Will still want to do the odd weekend / non-commute road ride though.

In short if it's a "quick release" type fitting then pannier could be the way to go.

Rack bolts on, Ortlieb panniers have spring loaded quick release so they will lift off when you grab the carry strap, but will not shake off.

They are very secure and come with inserts for different size rack tubes so they don't rattle or pound on the rack and fatigue the metal at the attachment point. Savvy design as you'd expect from the Germans.

Mine are roll-top and water proof so no fears foe the gear inside in bad weather. They survived my infamous crash in 2009 with just a slight scuff, so pretty tough.

make sure you can fit a rack to your bike. Most hybrid bikes will have mounting bolts, but high-end MTBs might not (particularly if rear suspension). the rack might come with clamps etc, but that's a bad option, IMHO.

might be best to find a cheap second hand hybrid or older MTB... that way you can store it without worrying AND use panniers

I use a Deuter Futura 30. It's OK with the mesh and frame thing that keeps it off your back a little. The problem with it is that if you are descending on the roadie in the drops your helmet hits the frame in the top of the bag.
I recently did a long mtb ride wearing it; carrying about 5L of water, shit loads of food, spares and tools and it was OK but not ideal.
The problem is when you have a load in it, you can't really ride out of the saddle at all. Even just standing up for the little pinch climb makes the bag sway and feel awkward.

Ive been commuting for about 2.5 years now on the daily basis.
I use an IT backpack. It is very solid.
I know some peeps don't like using backpack for various reasons. Weak lower back. sweat in summer & so on.
It needs to be solid, have lots of padding on the back (as already mentioned) & the straps as you will be carrying significant weight. yes mine gets pretty skanky but it just goes through the wash on the regular basis.
I carry up to 15 kg depending on day. Monday mornings & Friday afternoons in particular.
I find a chest strap important. It just helps distribute the weight a little. or stops a lot of the weight going to the lower back.
I have 2 back packs. The solid one & a lightweight one for the days I do intervals to keep up more with the mad-hatters. Just for the basics. Lunch, towel, wallet & stuff for a flat.

i can't tell what brand mine is unfortunately. Its bright red. That's all I know.But it cost me about $80 on sell out(was $150). so its prolly no longer available anyway.

The most important thing for commuting are disc brakes - simply for Sydney weather & drivers. Then good lights.
After that do whatever you can to leave your shoes, toiletries, etc at work. Get a locker, leave a bag in a cupboard do whatever. Commuting is so much more enjoyable without lugging these heavy & bulky things in each day. If you don't have a second pair of work shoes, belt, etc I'd say spend money here before you buy panniers.
After that the bag v panniers doesn't really matter.
I have a Deuter 32 AG. Pros = it's bullet proof, great rain cover, mesh keeps it off your back & a nice little zip pocket on your hip for keys. Cons = no separate laptop section & as above on the roady it comes up against your helmet.
I ride a hard tail with slicks. Nice to be able to jump a gutter or go rough on the odd occasion it's required. Not sure you can do that with panniers.

Checkout chrome bags.

I have the citizen. I used it on a motorbike for 3 years with a laptop and documents in it in all weather conditions, no problems at all. I have since used the bag for a daily bike commute for the last 3 years. Even in torrential rain my work clothes stay dry. It looks the same as the day I purchased it. My Deuter air Mtb bag is awesome but it can't hold a candle to the quality of the chrome bag.

Yeah another strategy sounds like the plan..... looks like the office I'm at isn't geared towards contractors who would like to ride to work (grrr) so access to bike storage / gym for showers etc isn;t looking likely.

I guess I'll need to start getting up with the birds and doing some pre-work riding.