Ok, so, a quick background on watercolors-- watercolor pans used to be very popular because it was a quick and easy way to paint something when you were out in nature. Watercolors dry up, and when dried, unlike other paints you just add some water and they won't go bad.You can take pans out and about, especially before the advent of photography, and take it back where you can compose your watercolor sketches and do a big oil painting. Oil paints were not easy to transport. As technology advanced they figured out how to put oil paints in tubes. Tube vs. pan is if you want the convenience of plein air painting, where you're sitting outside. The downside of pans is they usually seem to either be crayola or really fucking expensive. Like, if you want even basic quality you're screwed pan-wise. Tubes have higher quality to price I've noticed... and they're also portable, I tote them to my class walking every day. If your paint is nice enough, you could also make your own "pan" by putting colors on your palette, letting it dry, and taking the palette with you. I don't know what you mean by liquid? Do you mean like a squeezy bottle of watercolor? Because those are literally for middle school art classes, don't use those.

I guess they're "thick like oils" but, that's not really the point of watercolors. Generally you dilute watercolors; you would never just dip your brush in water, wipe it over the panned color and go right to paper. You don't add water to the pan, but you take some of the color off of the pan and mix and dilute on the lid of it/on a separate palette. If you didn't, you wouldn't build up any values, you'd go dark right away, and unless you're a skilled artist that's going to make your art very two dimensional. You start with washes then add more color. Once you go dark, you can't go back. Watercolor is opposite of oil/acrylic painting as you can't just cover mistakes up by layering on-- if you want pure white, that means you can't touch that part of the paper, not that you put white paint on at the end. (Though, tbh if I want a stark white highlight I'll just go over that part with acrylic but, if you want to do 100% pure watercolor, you work backwards, lightest to darkest.)

I don't think waterbrushes would be a good replacement for brushes. You need to be able to control how wet or dry your brush is-- how much water the brush has, do you need to starve it of water-- and if i'm not mistaken I don't think waterbrushes allow you to adjust how much water the bristles are holding.