I have some high res scans of floor tiles, not many though.
Does anyone know of any clever ways to create more variations of a individual tile without simply photographing more of them which isn’t an option at the minute? i.e. can I feed an image of a single tile into say Midjourney or Substance and generate different versions of it to then use as part of a multitexture setup?
You could try with MJ using the /blend option and use --tile --ar 1:1? You might get some interesting results. 
A quick and dirty way I usually do it is like this:
Make the textures tileable
Load them into multitexture and bercontile and randomize -1 to 1 in U and V, plus randomizing whatever scale and rotation I can without it looking weird. You can also do a slight hue and/or gamma randomization to break it up further.
This is usually enough, depending on the type of tile ofcourse