There are waterproof single side panniers which attach and detach quickly, have a rectangular shape meant for A4 binders and/or a laptop and have some inside pockets meant for office supplies.
An example is the Ortlieb Office Bag which I got when I was still in school, to replace my school backpack. It's an older model than the following picture but pretty much like it:
It has a padded detachable shoulder strap and a nice handle too.
When going on vacation it's also nice to have one bag like this, it doesn't fit as much as the usual round literal sacks but its content is much more easily accessible.
You get used to the imbalance, it's only really a problem when the whole bag is filled to the brim with books (like on the last day of school when bringing back all the borrowed books instead of just the ones for the day. In such cases I would use two panniers for once). The tires wear noticeably more on the side opposing the bag, so much that I once ran a tire backwards to get the last couple of hundred kilometers out of it (it was slick by then so the direction didn't matter).
I can very much recommend Ortlieb bags, they are expensive but last for decades. I still have that same pannier I got for school and it's still my favorite everday life pannier.
I also tried (in the shop) a convertible backpack/pannier back then and it took me too long to convert back and forth, but maybe they have improved since.
There is also an backpack adapter for all Ortlieb bags which I want to try at some point, but haven't yet.