You ask for input about a backpack.
My suggestion is a Chrome Citizen messenger bag. If you google you will find their website. I have one of the Nite series ones (with reflection built in) and I am very happy with it for use both on and off the bike.
I am surprised at how useful and comfortable it is. They have worked on the design for 20 years, and nothing falls out. Also there are numerous pockets and compartments inside it.
It seems large but it doesnt look so large on your back because it lays flat, and the size helps distribute the weight.
Before i got it i used a backpack designed for a Sony laptop, and it was a lot more uncomfortable and ungainly on the bike.

IF you get one of these Chrome ones you will not regret it.
They also have backpack models too but i have no experience with them.

I was looking also at bags that convert to panniers ie the Ortlieb Office and Vario models and the Arkel Bug model. I still may get one of these but i havent put rack(s) on my bike(s) yet and I like the messenger bag approach because i like to ride a few bikes I own none of which have racks.