Old grout / adhesive not shifting - opinions & thoughts

Hi guys, thought I'd sign up here and seek some professional advise.

Moved house not long ago and now working my round the house removing traces of the previous owner's questionable taste!

I'm onto doing the bathroom now - plan is to replace the toilet, sink and tiled floors. Bath to remain (for now).
I've removed as much tiles as I can so the sink and toilet are still functional. Problem now seems to be removing the grout / adhesive from the back of the old tiles from the floorboards. Some of it has come up but there are sections that are absolutely refusing to budge - have tried a chisel, scrapers, a Bosch PFM 180E tool and I'm getting nowhere fast.

Thickness of the old grout varies from 1mm to 3mm at the worst parts but I could probably reduce this down to between 1 and 2mm.
Photos of the offending room below:

General view of bathroom.



Worst looking area (around 3mm thick)




I have 600 x 600 x 9thk Porcelain tiles which I plan to lay down.

Floor area to be covered isn't spectacularly huge either, only 1500 x 1700.

Question I have is would I get away with fixing 6mm plyboard over the floorboards and the existing grout / adhesive then laying the 600mm tiles on top or would there likely be too much flex and movement?

This existing grout / adhesive really is a pain to shift and the only other option I can think of (which is a bit extreme) is to remove the existing floorboards completely and starting from scratch.

Your thoughts and opinions much appreciated.

Pie

Looks like chipboard from the photos?

you wont get away with overboarding with 6mm ply. 6mm backerboard would work if you get the adhesive up.

Try an SDS drill with a wide chisel bit - doesnt matter if you damage the chipboard. then overboard with 6mm no more ply, or take the chip out and repace with at least 18mm ply

Try a hot air gun, but don't burn the floor. It works well on walls.

Andy

overboard with hardie backer-board trowel over flexy adhesive to fill the voids then screw the board down ( where to screw is marked on the boards) then tile and defects (bounce) in the floor must be sorted first

simplest answer would be to remove the floor and replace with WPB ply making sure it is screwed down well with little if NO movement defection in the floor.

there is a right way and a wrong way,do it right, do it once, do it the wrong way and do it a couple of times, simples...........

Is it a ground floor b/room...or upstairs?...

Thanks for the feedback and suggesiton guys.
In the end I decided to just rip up the floor boards and start afresh.
I borrowed a work colleague's circular saw and removed the majority of the old boards - avoided going too near the walls.

Some photos of before, during and after.
Got the last coat of paint on 0130 on Christmas Day so was cutting things really fine since I was hosting this year!

Before





During




Toilet gone


Basin gone


Boards coming up
I'm not particulaarly great cutting a straight line as you'll see later on.




Replacement boards down
As you can see there are some gaps between the boards!


WBP Ply screwed down every 150mm (or thereabouts)


Tiling started




Tiling complete


Grouting complete and toilet installed
Noticed a very small leak where the service pipe meets the cistern inlet. Checked, rechecked and checked again that the rubber washer wasn't warped or damaged but was still getting a drip every 2 seconds with the water turned on.
Googled the problem (seemingly quite common) and replaced with a fibre washer; leak stopped!


Basin and Stand installed


Final Product
Wall cabinet and shelf unit from Ikea.