While the below comments describe the perfect solution, even just the ability to add a single customisable category would a huge benefit. Being able to customise the category order would also be very helpful.

Categories should function just like folders, but basically they have some preset filters for them. So in that case, we should be able to add any folders to the top as a category. The big one I want is a "finance" one for all banking and credit card related items.

Agreed. To be clearer, I would suggest:
- Being able to rename existing categories
- Being able to create new categories
- Being able to assign custom or preset colours to each
- Being able to define a logic for how emails are captured into these custom categories (maybe even using Lumo)?