One for Kevin

Ceramic tile approx 40 years old in a cafe formally a wimpy burger bar. Owner cannot get it clean. I dropped in whilst the place was open so a machine was out of the question - trailing leads etc. Had a quick play with high alkaline and a bit of 220 honing powder. Made it slightly better but this tile is textured for non slip purposes I presume and it was not shifting. Going back with the machine for a real test. Method will be; wet out, high alkaline, dwell up to 20 minutes, honing powder, rotary machine, wet vac, rinse, wet vac.
Anything else? Bit concerned about this one as the test by hand was not impressive but I would guess dwell time was not enough??? Had a little dabble with solvent based coating remover just to see what that did and the high alkaline was best.
Put a little bit of acidic brick/mortar clean up solution out of interest - did nothing.
100 square metres of this stuff to do - photos attached