If you have a suspended floor on joists, watch out for the spaghetti underneath.
I had to take the whole floor up - there were 15 misc size bits of all sorts of timber, revealing evidence that the back of the house had been on fire!
I was going to do a course of tiling wooden substrates but when I’d finished this bathroom I discovered I’d done it all according to the book anyway.
First, w/proof flooring particle board - superior chipboard. In UK is always green
Next, a layer of 19mm waterproof ply. No need for ‘marine grade’ [which hardly ever is]. Shuttering ply - the stuff they use to make formers for concrete and put round building site as fences - perfectly good.
Keep scouting the oddments pile in the tile warehouses. I was lucky to find a few boxes of natural slate at £8/m. It is [minimally] porous, so a couple of coats of sealant …
Flexible adhesive - job done.
It’s not difficult to do but the pros are quicker.