This isnt a power bank, but it can charge any external power bank or your actual phone directly. Advertised at 30w and can fit on your backpack
About 6x more watts charging than the 6w thing you found. Meaning, in cloudy weather it could mean having a constant charge vs no charge at all.
Now for 3x the price, you can get 3x the amount of sunlight you can harvest with this other foldable one. When folded its the size of a paper so you can also fit it comfortably in your backpack without sacrificing too much space and weight. This one should actually be able to charge your device and power bank a lot faster than the others.
Its up to you if you wanna sacrifice money and space for significantly faster charging