I agree. Without subcategories it’s hard to see how cash & credit card payments categories can work. Hugo’s explained the challenges with that functionality here.
But I expect Monzo would rather you spent directly on your debit card..personally I try to avoid doing anything but that for this reason (there’s lots of benefits if you do), although it’s not always possible.
FYI this is what they intended the Finance category to be used for, although you’re free to adopt your own definitions of course.