Another one for your list: The Tascam US16X08
https://www.sweetwater.com/store/detail/US16x08
But I agree with the comments about your recording/mixing environment - If you can't get decent recordings using the R8, why not? What are the problems?
Room treatment (not foam) is one of the best investments a home recordist can make as none of us have the best recording environments. The ability to 'record what you hear' and also to accurately monitor for mixing purposes are crucial.