Best subfloor for Classic Limed Oak SCB-KP97 Glued?

For this type of job, I’d not choose between 6mm plywood and self-levelling compound until you’ve exposed the existing subfloor. The correct preparation depends on whether you end up with sound timber boards or concrete. For the concrete area, I’d remove the old flooring and adhesive, check for moisture, then use a suitable DPM/primer system if required, followed by a good-quality cementitious self-levelling compound to get a flat, stable surface. Then use the flooring manufacturer’s recommended adhesive for the glued-down KP97. For the timber/wooden plank areas, I wouldn’t normally pour self-leveller directly onto old floorboards. If the boards are structurally sound, screw them down properly and overlay with plywood (typically 6mm, subject to the flooring manufacturer’s requirements), with joints staggered and securely fixed. The surface then needs to be flat enough for the glued LVT/wood flooring. The important point is that glue-down flooring is much less forgiving of unevenness than click flooring, so getting the subfloor flat is more important than simply choosing one particular product. In the UK, for example, you could look at systems from UZIN, Mapei, Ardex or F. Ball. I'd also check the KP97 installation instructions before deciding on the exact primer, smoothing compound and adhesive, because the flooring manufacturer's requirements should take priority.