Underfloor heating prep/base layer help!

Hi,

So I can’t find any solid information on this topic. For context, I am wanting tiles floor with underfloor heating from the kitchen into the living/dining room (joining rooms via a door) so basically the whole downstairs. While having a new kitchen installed prior to tiling I was advised to lay some insulation backer boards directly onto the existing floor boards by using flexible tile adhesive and screws with washers. I am now at a stage where I can continue laying the insulation boards through to the living room ready to then continue the process of, electric UFH mats, a self levelling screed and then tiles. My issue is, after laying the boards in the kitchen directly onto the floor boards, the living room has plywood sheets down to which I now am thinking would of been better to lay the insulation boards on to as oppose to directly to floorboards. So what’s the correct way from now? Do I lift the ply in the living room and carry on with the adhesive and boards, or have I messed up already in the kitchen and need to track back. Any help would be greatly appreciated, thanks!