Flattening stone should be larger and much coarser than water stones. Wet 100 grit SiC sandpaper on glass works too.
DMD 150/400 10 x 2.8" diamond lapping plate, 27$, 2 year warranty. https://smile.amazon.com/DMD-Double-Sided-Sharpening-Whetstone-Professional/dp/B088ZHRYDF
Coarse Silicon Carbide sandpaper. https://smile.amazon.com/gp/product/B09KZHJ8YV amoolo Wet Sandpaper, 40 to 100 Grit - 9x3.6 Inches (40 Pcs) $5
Lapping with coarse SiC grit powder and water on hard flat surface (glass, tile, cinder block) retexturizes hard glazed stones. https://youtu.be/NLG2N7duwYc Stone Maintenance - Flattening and Conditioning Sharpening Stones. Jef Jewell