Well light and big is kind of at odds, capacity is basically going to be proportional to size and weight (...barring companies lying about their capacity). Like at 20,000+ mAh you're looking at a pretty thick and wide bar format for the most part, with a few thin but much wider/larger footprint slab options. If you really want light (granted "light" is relative) I think you'd end up looking more towards the ~10,000 mAh batteries.
Best options would be ones with USB-C and the USB Power Delivery spec cause they can charge the fastest (the big USB-PD Anker and RAVPower ones are some of the few options iirc), otherwise just USB-C itself should in theory be enough to always charge, if a bit slow (lots of options, not sure which are good/bad though), while USB-A will vary (but gives the most options by far of course). Like within Zelda it can range from draining, to staying level, to charging, depending on what's happening/what area you're in.
I got a Jackery Titan S myself, but use an A-C cable for now. Mostly just for the convenience of not having micro USB. Also found out it does pass through charging which is nice, although not that big of a deal with how many charger ports I have.
...and if you ever want to power the dock portably I think you specifically need something that can do the 15V/3A USB-PD profile. The Anker and RAVPower ones only do 15V/2A there if you're curious. There's a Xiaomi one that works iirc but I'm not sure where to buy it outside of eBay and other Chinese seller sites.
(Course USB-C and PD is kind of a mess where it seems like most stuff isn't technically compliant but works, including the Switch itself)
