I think a 10-20 liter pack is the sweet spot for activities like outdoor festivals.
Enough room for gear and snacks, a water bottle and even a jacket.

Agreed. The one I use the most when outdoors is an older Camelbak that I think is around 12L. With the hydration bladder full, I have room for a trauma kit, a boo boo kit, a ham radio HT, PSP, knife, sunscreen, bug spray, small multitool and a few other odds and ends, with the main cargo compartment being about half-empty to any accept activity specific items.

The smallest regular backpacks I have are around 18 or 19L which is a nice size for a day pack. I don't do festivals, just hikes and camping trips. On a day hike I still like to have the minimum stuff I'd need for an overnight stay; to me that's pretty much the definition of a PSK.

Anything more than about 20-23L and I'd want a proper waist belt that takes some of the load, so a small bag like the Trident 20 is about perfect for day trips / overnighters or misc light work IMO.

Friends swear by this: https://www.rei.com/product/148592/rei-co-op-flash-18-pack

I just ordered this in 10L since it's on sale. I'm a big fan of their bomb-proof products!

https://www.goruck.com/rucksacks/bullet-ruck/

The Bullet is almost the only GoRuck bag I don't own. Very tempting at this price!

Comes in a variety of colors and prints and in a 22 l version with more features

Problem is....most 'festivals' now limit things and don't allow more than a woman's clutch size bag.

I went with the REI Flash 22. When I compared the two the slightly larger Flash 22 had three exterior pockets besides the main compartment which makes for better organization. Still lightweight and simple.