And is there any grout that is not carcinogenic?? I'm looking at the safety specs of these products, and they are full of warnings... but no info on if the danger is mainly during application... how long effects may last, etc.

Your professional tile contractor is the one that should be telling you about the best product for your job. It’s no use specifying something that be won’t use. Find your tile pro first.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

My tile contractor is okay using anything, but has no experience with epoxy, which rules that out. So I might as well go with one that seems like it is better than the cement-based grouts at being true to sample colours, and is less carcinogenic than others... landing on Mapei at the moment.

Flexcolor CQ is a fine product in the right hands. It does require a more intensive cleanup, and installers generally charge more for the extra time involved. If your Pro has recommended that, and has used it, that would be your answer. But check with him first.

Yes, I was thinking of Flexcolor as it works with 1/16th grout lines too. Will certainly check before getting it - last thing I want is to be a guinea pig. Cheers!

But does 1/16 grout lines work with your tile choice? Is the substrate excruciatingly FLAT? Is the tile rectified? And does the installer have the skill and use a lippage tuning system?

It all comes back to your choice of installer.

Yes, the tiles were chosen because they were rectified, and the sub floor is being newly installed and will be as flat as they can get it. On top of it will go a couple of different types of thin sets along with Ditra. What I do not know is if a lippage management system will be used.

I'm wondering if a medium based mortar should be used over the Ditra rather than thinset as we're dealing with large 19x19 tiles... thoughts?

1. Epoxy grout is not shiny: it looks like regular grout. But it does require someone with experience to apply it properly because it is very stiff, dries quickly, is harder to force into thin grout lines, and is difficult or impossible to remove if it dries on the surface of the tile.

2. The main health risks from grout are to tilers who don't wear respirators when mixing up the dusty compound, and a slight, temporary offgassing as it cures. For your purposes, grout does not pose any acute or long-term carcinogenic issues. There are thousands of common household products that contain chemical compounds with carcinogenic potential, but that doesn't mean that you are going to get cancer from them. It's wise to be aware of potential risks from various products. However, if you read every product's MSDS sheet, and don't have an epidemiological and risk management background, you can scare yourself for no reason.

3. Grout immediately. Ungrouted tile can result in chipped edges, particularly if you are having people working around and walking on these tiles. I also hope you intend to protect the tile surfaces so they don't get damaged from drywall sanding grit and traffic in the area. I would choose to lay tile after that work was done, but, if you have no choice, make sure the floors don't suffer as a result.