Thank you for not ruining nice new concrete. There's actually 2 approaches I would consider for the floating floor much better idea.
First is, to use battens (spare plywood strips or thinner plywood strips) to attach the plywood to each other & not have Celotex under those, vapor barrier yes but not Celotex...unless you can get thinner Celotex. Or, to use tongue & groove plywood to avoid battens altogether.
Second is, to use cement board since it won't warp or cup over time. These seams would just be taped & cemented or could just be left loose until & if a problem ever reveals itself. CB will also have the ability to crumble if expansion & contraction of the Pine battles it's confinement & it will.
I think the real benefit of large panel backing would be to pre-fabricate complete panels, if possible to handle for the room size. I'd build it upside down, would allow for sloppy or loose holes in the plywood for movement of Pine. Dry-lay the floor boards with good book matched or vein aligned or designed face down, with wax-paper strip spacers tucked under the good face (for removal later).
Slather on glue or even contact cement, drop on plywood or CB & screw through it & into the Pine, 25mm screws so future re-finishing can be performed at least twice. Edges of pre-fab panels would be overrun with Pine on one edge & run-short on the other edge to provide an overlap or under lap...same for ends depending on orientation & length of run.
Completed CB panels can be slid into place with short-run face painted with glue or can be edge glued. Plywood panels can be edge glued or glued to free batten edge & overrun underside.