Good progress. You certainly need good ventilation with the epoxy coat don't you, I nearly gassed out my family indoors even though I did the work in the garden!
I think 4mm is enough for one mirror grind, I still have loads of depth remaining after reaching the polishing stage. 10mm should last several mirrors I should imagine. I don't know if you want to use this trick, but I melted wax in between the tiles on the tool so you can scrape the wax out after each grit size and then re seal it with wax. In theory this should help stop any grit which is lodged in the epoxy coming loose and scratching the mirror once you've moved to a finer grit.
One of the fine gents on this thread told me this and it has worked well so far
Good luck with the next stage, and I personally think it would be better if you can get the glass in smaller bits for more randomisation and extra channels whilst grinding. Things get caked up and stuck sometimes!