Why not pushing thickness to match L12S in pushpull of slim fan? ie 85/86mm?
Are you not fearing to recycle hot GPU air in cpu cooling while just using L12S, 70mm tall with 79mm wide case?..
Having space in front of the fan instead of pushing it right up against the grill has quite large noise benefits. Fan noise can be very bothersome when positioned in close proximity to inlet grill.
As long as you can evacuate the air efficiently, I don't think CPU cooling will be hugely affected by GPU thermals. Adding a fan in the bottom 120mm would help as well I think. You can look at HW Canucks' recent review of the P-ATX v2 for evidence of this. They used a 5950X and RTX 3080 in the P-ATX (much smaller than N-ATX) with a 59mm tall cooler and still got good results.
If you do find CPU cooling inadequate, a very popular solution is to 3D-print a fan shroud or make one out of cardboard to ensure the CPU cooler ingest air from outside the case.