I have used a DMD 400 grit 6x2" diamond plate for stone flattening and sharpening for over 20 years. I have also been pleased with their 4x1" pocket stones and blue 10x3" lapping plates. Other options:

UltraSharp diamond plates (<3K grit) have a lifetime warranty. https://youtu.be/npEubLayJ7k Step by Step Freehand Knife Sharpening Easiest Way. Neeves Knives

CKTG 140 Grit Diamond Flattening Plate 3"x8" (4.5 out of 5 based on 10 reviews) $29.95 https://www.chefknivestogo.com/140grdistflp.html

WorkSharp GSS 6x1.5" diamond and ceramic kit. 1 min. https://youtu.be/Wus94N-tcRk Work Sharp Guided Sharpening System

Chisel in jig on cheap diamond plates set. https://youtu.be/f4BEQeW8cdo Stop Sharpening with Sandpaper! -Rex Krueger. You can ditch the sandpaper and get sharp with affordable diamond stones.

https://smile.amazon.com/dp/B09M9TPWWQ Chumia 5 Pcs Diamond Sharpening plates, 5.9 x 2.48" (200, 400, 800,1000, 1200 Grit) 4.2 out of 5 stars 97 ratings $20.99