I do agree with the Dior suggestions; they are really putting out some great looking bags.
You mentioned a bag that can be used for night time and still be fun and for daytime use
https://www.fendi.com/us/woman/bags/p-8BT286A388F13VO
I would go for the medium but simply as I don‘t like bags to be stuffed (and look the part). I am quite positive that you could fit everything you mentioned in a small one tho...
A small will do but @Stansy is right that the Cuir shouldn't look like it's too stuffed. I think 2 important variables here are the size of your tiny umbrella and how small your bottle of water is. If it's say a tall 600ml bottle, it won't be able to stand up in a small. Is there anywhere you can go to try putting your stuff in a Cuir to get a better idea of which size is best?
Thank you both for your help! I'm looking for a lightweight leather work bag & based on your advice I've ordered the small. It arrives tomorrow- I'm super excited! I considered getting a customised version but the delivery time is 7 weeks & I need it much sooner than that which is a shame