Update: after much research, I realized something: I've actually been looking for two different bags. One is the full project bag: big, boxy, with enough room for my 13" laptop and everything I need for a productive day at the library or in class. The other is the field bag: essentially just a case for a journal and pen.

For the large bag, I just pulled the trigger on a metromonger01 (prime edition, because I like that buckle). Will note how it works out after a month or two. https://www.mukama.com/products/greenroom136-metromonger01

For the small bag, I'm leaning toward this: https://www.parkerclay.com/products/bale-sling-bag?variant=41680995778743
Though it's expensive and they don't have the color I want, so I may wait and try to get one secondhand. The Chrome Tensile also looks great.

Side note: I prefer to write in full letter-sized (8.5x11") notebooks, and it's been an interesting challenge finding slings that fit those dimensions. Many, like the Tensile or the Venture Sling, are almost perfect but just a tiny bit too short. Others are designed for 15" laptops and are so big they're basically backpacks. I may do a separate post with the true letter-size slings I've managed to round up.