The trick to finding big patches of Phyllite with ease is just digging in straight lines at y level 4-6 for thousands of blocks in one direction (ive found that almost all of my Phyllite deposites were located under bodies of water). When you dig a few thousand blocks in one direction at y4-6, start an adjacent strip mine 500 blocks to left or right and continue back and forth in a zig zag pattern. The reason this works so well is because Phyllite spawns in large chunks of 400x650 compared to Kimberlite (which only generates in small chunks of around 60x70 blocks) so finding Phyllite is relatively easy if you smith around 8 or so steel picks and just bring temporal gears to regain sanity whilst at the mantle layer, it just takes a little time. I have found around a full trunk of Phyllite rocks worth using this strategy and have use around 12 steel picks per Phyllite chunk I find.