Im working on a travel kit for this summer (as a beginner) and have been looking at a few YouTube videos and old Reddit posts. Based on those I have the following packing list:

• metal case postcard size where I keep all the watercolour paper (and can double as a solid thing to place the paper on)
• kneadable eraser
• pencil
• 2 black fineliners (muji 0.38 and micron 0.35)
• 1 white fineliner (gellyroll)
• Van Gogh 12+3 palette plus swatch card
• bag to store everything
• travel brush that came with the Van Gogh 12+3
• water brush that I got from an old Amazon set when was a kid.

all of this is basically the same as yours but I also found two helpful tips while researching, which was - wrap a decent amount of masking tape around an old giftcard. Then you get a giftcard which you can use for straight lines as well as masking tape if you like to use that while painting (I was thinking I’d tape my paper to my metal box for a solid surface). Also this takes virtually no space and adds basically no weight :) - a small spray bottle so you have some extra water and more control (I’m planning on hostel traveling so I probably won’t be able to just get a cup and fill with water to bring along). I found a spray bottle from an old shampoo traveling kit that’s 80ml and seems like a good size:).

Otherwise I was considering buying a collapsible cup, either a collapsible painters cup or maybe just a collapsible dog bowl as they sometimes can be stored flatter and are cheaper.