Shower room refurb flooring preparation

So, I'm refurbing a small shower room which is around 2.8m x 1.6m overall. Walls tiled about 4 years ago and all good.
The floor is mostly concrete except for two channels approx 100mm wide on the long, side walls, which carry radiator, sink water pipes on one side and shower drain waste on the opposite side. These channels have chipboard covers flush with concrete.
Shower base and cubicle now done and old WC and sink about to be removed and new fitted after floor done. New flooring with Karndean T88 Onyx vinyl, glue-down tiles to be fitted so need to prep the floor and do the tiling before fitting new WC and sink.
Originally intended to fit 6mm ply over the whole floor to provide a new, consistent base, but although the concrete floor is dry with no sign of damp etc, I'm wondering if it's the best solution (possible future damp and swelling of ply when 'sealed' over and not 'breathing'??? )
I also do not want to remove existing skirting boards and risk wall tile breakages etc which are only a few years old, so more precision needed if I fit ply and then floor vinyl tiles.

Maybe a better floor preparation solution would be:-
1. Seal/cover the concrete-to-chipboard joints with (????) tape (Anything special needed?)
2. Apply a SBR primer if necessary?? (1-2 coats suitably diluted.....any recommendations??)
3. Apply SLC to, say 4-6mm, (nominal thickness only as concrete floor level and condition is very good) to provide new surface.
4. Fix Karndean tiles with their adhesive (no need for further priming or sealing of the new SLC-covered floor??)

Appreciate views please

 

Hmmm.....if i have understood you correctly, I have used aquapanel boards to line the plaster walls inside the shower, so you suggest using these on top of the concrete floor. Did you 'seal' the concrete floor with PVA or something else before fixing (Glue or ??) the boards.