I fostered my friends XXL kitty for 5 months a couple years ago. I took him to the groomers at least once, maybe twice. Once I took him to the vet to get his nails clipped and the tech was nice enough to cut the mats from his rear for me. I was so grateful! That is the only place he matted was on his bottom. I didn't know him well enough to tackle it myself. I think my friend takes care of his fur on her own. But he loves her. LOL
I would brush him when I went in to visit him in his room. (he terrorized Paizly so he was shut in a bedroom unless under closer supervision)

Sorry I do not have any good suggestions for you. But I hope you can find a solution. I am horrible and brushing the dogs regularly. So I'm glad to have short hair thin kitties (at least for now!).