help with adhesive for large encaustic tiles

[almac39]

Hello, I'm a new member and DIYer hoping that you can give me some advice on a tiling job in my new home. I am laying a hall floor with 8" encaustic floor tiles, 21mm thick, apparently originating in a French wine-merchant's chateau (they were quite expensive so I NEED to get this right). They seem to have a porous cementitious base. I have prepped the floor by removing the boards, putting in robust noggings at 500-600 centres between joists (which are approx 300 centres), and overlaying with 18mm exterior grade ply screwed at 150-200 centres. As such the floor is about as rigid as I can get it, feels solid, but is still suspended so there's going to be a element of deflection, I suppose. Not sure to what extent I need to worry about minimal movement?

Can anyone advise me on the best adhesive, bed/joint depth/width, or suggest anything else that I might do to protect the tiles?

I'll be very grateful for responses,

Cheers

Al