Yea, you got it. The 400 will get the stone flat, your Toma Nagura will make it smooth, as will just about any higher grit stone, your 1k? You can feel the stone face get smoother as you rub it. Or you can just hone on it and it will smooth out. A lot depends on how aggressive your plate is, how clean, and how much pressure you used with the plate.

You can raise a slurry with a diamond plate, then raise a thinner slurry with your Toma to finish. It should be finer, but you have to experiment.

Most new diamond plates do need to be smoothed, as the diamonds can be irregular and may have some high diamonds that can scratch. Rubbing some steel, a chisel, sharpen an axe, Kitchen knife or lapping a couple stone will usually smooth it out and it is good from there.

If down the road, your diamond plate feels like it is not cutting as aggressively, scrub it with some cleanser and a stiff brush, let the cleanser sit on it for a few minutes, I like Bar Keepers Friend for this, makes a big difference and will keep a plate cutting for a long time. I have some plates over 10 years old that still work well. Using too much pressure or without lube, water for stones and Windex for steel, or water, will rip the diamonds from the substrate. But if you use a lubricant, clean them and go easy on the pressure, they will last a long time.