I have similar problem with Nelli, as she is also either a bit chubby or just plain lazy… I tend to wash her butt with just plain water if it looks like washing will help. Sometimes I only cut the fur, and sometimes I have to do both. I have noticed that if I keep her tummy fur shorter (she is mixed lop and lionhead so she has lots of fur) and the butt area*s fur even shorter, she is able to clean her butt better by herself. She doesn’t smell really, and her scent glands are usually very clean, so I think the rinsing her butt and keeping the fur short is helping. (I don’t rinse it every week, more like when really needed)

It’s strange though, Malla, Nelli’s sister, keeps her butt so clean, that the fur is always white! If it gets a tinge of yellow I cut her tummy fur as well, and the fur is white again the next time I check. But, she is smaller in size than Nelli, and even seems to curl up like a ball when cleaning herself. Nelli probably really is lazy haha.

Bindi has long fur and often gets little dirty mats in that area. I use a mini-trimmer (made by Wahl, in the pet section at Walmart for around $10.00). Works great and quickly and you don’t have to worry about cutting skin accidently.

Rumball too. Plus he has longish fur in some areas underneath.
I use small scissors from a first aid kit that have rounded ends.
This mini trimmer sounds useful!