2nd floor attic type toilet install

[Edorrjr]

I have a chalet style house. There are knee walls with 4 ft distance to edge. In that knee wall is the waste stack going up to the roof There is also hot and cold water run up there. Easy access. I’m putting in a dormer to give head room. I plan on having just a tiny sink with small toilet to add second toilet for the house. I know you need 1/4” per foot for drain. The stack is 8 ft away. I originally thought about doing a raised floor to accommodate a waste drain but see these rear drain toilets with p traps. Some are 4” some are 7 ish.
Would this be a good option for this application? Anyone do similar?