Waterproof luggage: I used a 12 litre Tupperware box, or the smaller ones of the set. They were long "celery boxes" for reduced air resistance. Totally waterproof. Translucent for easy finding of stuff, also I sometimes put a big reflective strip or lights inside. It held dress shoes & pants & shirt, manilla folder, lunch. One bungee held it fine but I used two for valuables just in case it slipped off unnoticed. A cord & tubing handle on one end would have been nice but I didn't do it.
The Topeak Beam Rack and similar products are great for commuting if you can afford them, though I find the stealworthiness of the beam rack is such a big problem I still use my trad carrier mostly.

Backpacks are sweaty and draggy, a carrier is a must. Get one with a filled in strip that acts as a partial mudguard. That can be extended with a stiff bit of EVA foam - no drag but not nearly as good as a real mudguard.

I use a monster bumbag too, low drag but uncomfy with tons of shopping in it, so on shopping days it often contains a superlight backpack.

Vibrations on the carrier will kill cellphones, scratch CDs, puree apples. Bubble posting envelopes are the solution. For a laptop I would use soft foam then hard foam then a rigid board underneath to stop the case from flexing.

Your lights and reflectors can be in danger of being covered by your parka or luggage, and when you are very cold or tired you can be dopey enough to not notice.
Generous use of white reflective tape on the crank arms is extremely effective and can not be blocked. If your batteries or brain fail, moving and blinking crank reflectors will likely keep you safe.

Motorbikes look as if they have just one tail light, but there are two bulbs and they do not share wiring. I routinely use two rear bicycle lights in case one dies, falls off unnoticed, or gets covered up.

Once you are set up properly, storms are a blast!