Quick-release saddlebag

That's kinda big for a saddle bag.
Maybe try a bar-mounted bag instead?
Should be easier to find one like that with a quick-release.

You might want to checkout "Topeak".
They have quick release saddle packs.
Although a 6L bag, I don't believe they would fill that need.

Sorry to hear the old bag found legs.

Good luck.

I agree with Dabac and Bikeyguy. The reason you can't find a quick release seat bag at 6 liters plus is because the quick release designed to small so as to fit under the seat, thus as being too small won't have the strength needed to handle a large 6 liter bag filled with stuff.

So combining what Dabac and Bikeyguy said is what I would do and in fact it is what I do when I commute. I actually use the Topeak Aero Wedge seat bag, these bags are so good I have two of them, one is a medium and the other is a large. Since they are only treated for water repellency you should spray them with Kiwi Boot Protector (yes it says boot but it works with fabric as well) when you buy it and again every year, and make sure before you spray that you unzip it so it expands to it's max size. This seat bag does have the option of a quick release system though none of mine have that because I never saw the point, Topeak uses two straps and a velcro strap to hold it against the seat post, the quick release still uses the velcro strap around the seat post, so for me the two straps simply unclip and I can remove and install it quickly. Anyways here is the quick release one: https://www.amazon.com/Topeak-Aero-Wedge-Pack-QuickClick/dp/B000CEMC3M/ref=sr_1_8?ie=UTF8&qid=1502162362&sr=8-8&keywords=topeak+aero+wedge+seat+bag

Then use I use a Topeak Tourguide DX2 handlebar bag that's almost 8 liters. Lots of handlebar bags on the market but I like the quality of the Topeak bags enough that I haven't ventured into looking at other stuff. It does feature a quick clip attachment, see: https://www.amazon.com/Topeak-Tourguide-DX-II-Handlebar/dp/B000BRC6TK/ref=sr_1_3?ie=UTF8&qid=1502162622&sr=8-3&keywords=topeak+handlebar+bag

I never take my bags in where I work because my bike is in a secure location and no one can bother it, but since your's is not you simply take both bags and your pump and lights you can put inside a bag then take it inside with you.