I shudder at the thought of lapping ANY hard hone/stone with a diamond plate.... I almost ruined my DMT 325 lapping plate on a couple of super hard barber hones back in the day. I stopped before I completely wore out the DMT, and it still works fine (but slowly) on typical man made hones like the Naniwa and Shapton Glass series, but I would not use it on a hard hone like the Zulu Grey and in this case it sounds like the Suehiro 20K is in the same league.
thebigspendur mentions the Shapton lapping plate, not to be confused with the Glass Diamond lapping plate or their other reference diamond lapping plate. It is just a flat piece of cast iron with a pattern in the surface and is meant to be used with loose SiC powders. This is the tried, tested and true lapping method machinists are familiar with. It's inexpensive and gets the job done, but it is messier than using a diamond embedded plate.
For simple/cheap, a flat concrete floor, a cookie sheet to contain the mess, some SiC particles and a bit of water will get the job done, saving your expensive diamond plates for what they do best, cleaning between honing razors on soft hones that load up with swarf.
DMT vs Atoma. I've used and own both.... I prefer the Atoma brand diamond plates. The difference is that while DMT evenly spreads their diamonds over the entire surface of their plates. Atoma makes little piles or buttons of the diamonds leaving empty space between them. Atoma plates suffer less from suction in use when flattening/cleaning hones.
I recently flattened the backside of my Zulu Grey 8" x 3" hone.... that is one hard rock and I promptly put away my diamond plates and brought out the lapping gear with loose SiC grit. I use the glass plate with plastic cover sheets from Lee Valley Tools. You could use just the glass plate, but you eventually dish it from lapping... By using the inexpensive plastic sheets, when it wears through, I just stop and replace it, but they are quite durable. I also mark the glass with a felt marker as to what grit I use on that side, then cover with the plastic and I don't have to worry about an occasional errant grit embedded in the plastic as it will always be the same size. But rest assured, even using SiC grit, that Zulu Grey hone took quite a while to flatten.
I don't worry about the finish of my hones..... the stria in the surface is just negative space and has no effect on the razors edge, unless it's super deep and at a 90 degree angle to the length of the hone. If the stria on the surface are large enough and deep enough to affect the edge of the razor, the hone would have to look like coarse metal file. I know some swear by "polishing" the surface of a hone, but that means you are knocking the tops off the grit in an attempt to reduce it's cutting effect. This is the method used with barber hones of yesteryear and today's Tormek water wheel grinder. The Tormek system uses a SiC two sided hone. The coarse side is used to rough up the surface of the wheel and expose fresh grit, and the fine side of the SiC stone is used to burnish and flatten the grit of the wheel without tearing the grit out of the surface, emulating a finer grit wheel. For what the Tormek is used for... it works fine, but for razors.... I dunno.... it's like taking a 1K Naniwa Chosera and after burnishing the surface claiming it is a 10K hone.... That just does not work for me.
So.... If it were me, I'd put the diamond embedded hones away, and instead grab some cheap loose SiC grit and lap that stone the old fashioned way.... I'm sure there are many sources for SiC grit, but if you are in a bind to find some, Lee Valley runs a internet mail order service and ships to the USA as well as in Canada. Shapton's loose grits are the same thing.... if you have a supplier of those, that's fine too. The only thing I would watch out for is the lapping compounds sold for lapping automotive cylinder head valves. It's the same grit, but it's mixed with mineral oil/grease to make it into a paste. You want the dry powder type and you want to use water for lubrication so it doesn't ruin your hone.
Regards
Christian