the same thing happens to my bun’s belly. I had to get her comfortable being held – I bribe with treats so I can lightly brush her belly while I’m holding her. That helps to get the mats out. She usually only has one or two tiny spots where her fur is matted, but being able to brush out that thick belly hair helps a lot in keeping the matting down.
Another thing I do is after I brush her I use a lint roller on her. Sounds weird but works pretty well. I just roll it on my shirt once or twice so it’s not too sticky and feed her treats while I lint roll her. It grabs onto a lot of the loose fur the brush hasn’t picked up. She isn’t bothered by it and will actually just sit and let me do it sometimes.
I think the best thing you can do with a fluffy bun is to just be sure you’re brushing a lot and getting them used to being brushed. Hopefully she’s just in a period of excessive shedding right now and won’t always be like this. I know Charlie’s shedding gets crazy for a few weeks and then will be fine.
Hope you’re able to figure something out! And sorry to hear your other buns are getting upset with her presence.

I’m not sure when the molt starts but seems like it’s possible. I can’t put Charlie on her back either. I hold her in one arm – like kind of scoop her and put her head kind of in the crook of my elbow and use the other hand to brush what I can of her belly. I of course can’t get it all but I get a bunch of hair off until she’s done and wants to get down. If you bun struggles with being held, I don’t know what to suggest other than to maybe try getting her used to and comfortable being held